Foundry Technology Unit 4: Gating and Risering System Overview

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F
OUNDRY
 T
ECHNOLOGY
U
NIT
 4
 
 
-Monil Salot
 
1
 
S
YLLABUS
 U
NIT
 4
 
Gating System:
Elements of gating system.
Classification.
Gating design considerations,
Gating ratio.
Pouring equipments.
Risering System:
Risering practice
Functions of riser,
Directional and progressive solidification.
Centerline feeding resistance.
Riser efficiency.
Riser design considerations.
Risering curves.  Cain’s, N.R.L. and Modulus methods,
Feeding distance and feeding aids,
Blind and atmospheric risers
Quality Control in Foundry:
Casting defects, their causes and remedies.
Shop floor quality control tests such as composition control,
Wedge test,
fluidity,
temperature
measurement etc.
 
2
 
GATING
 
SYSTEM
 
The 
term 
gating system 
refers 
to all passageways
through 
which the molten metal passes to enter 
the
mould
 
cavity.
The gating system is composed
 
of
Pouring
 
basin
Sprue
Runner
Gates
Risers
 
Components 
of Gating
 
System
 
Any gating system designed should aim at 
providing 
a defect
free 
casting. 
This 
can 
be 
achieved 
by 
considering following
requirements.
 
A 
gating 
system 
should avoid 
sudden 
or 
right angle changes 
in
direction.
 
A 
gating system should fill the 
mould 
cavity before
 
freezing.
 
The 
metal should 
flow 
smoothly into 
the 
mould 
without 
any
turbulence. A turbulence metal 
flow tends to 
form dross 
in 
the
mould.
 
Unwanted
 
m
ateri
a
ls
 
such
 
as
 
slag,
 
dross
 
and
 
other
 
m
ould
materials 
should not be allowed to enter the 
mould
 
cavity.
 
The 
metal 
entry 
into the mould 
cavity should be properly
controlled in 
such a way 
that aspiration of the atmospheric 
air
is
 
prevented.
 
A 
 
proper
 
t
h
er
m
al
 
g
rad
i
ent
 
should
 
be
 
m
aint
a
i
n
ed
 
so
 
that
 
t
h
e
casting is cooled without any shrinkage cavities or
 
distortions.
 
Metal flow should be 
maintained 
in such a 
way 
that no 
gating
or 
mould 
erosion takes
 
place.
 
The
 
ga
t
i
n
g
 
system
 
should
 
ensure
 
t
hat
 
enough
 
m
olten
 
m
et
a
l
reaches the 
mould
 
cavity.
 
It
 
should
 
be  
economical  
and  easy
 
to
 
implement
 
and 
remove
after casting
 
solidification.
 
SYSTE
M
,
 
TH
E
 
For
 
p
r
o
p
er
 
f
u
nctioning
 
of
 
the
 
gating
following factors need to 
be
 
controlled.
 
T
y
p
e
 
of
 
p
o
u
r
i
ng
 
e
q
u
ip
m
ent,
 
such
 
a
s
 
l
adles,
 
po
u
ri
n
g
basin etc.
Temperature/ 
Fluidity of 
molten
 
metal.
Rate of 
liquid 
metal
 
pouring.
Type 
and size 
of
 
sprue.
Type 
and size 
of
 
runner.
Size,
 
n
u
mber
 
and
 
lo
c
ation
 
of
 
gates
 
co
n
necti
n
g
 
runner
and
 
casting.
Position of 
mould 
during pouring 
and
 
solidification.
 
POURING
 
BASINS
 
A 
pouring 
basin makes it easier 
for the 
ladle 
or
crucible 
operator to direct the 
flow of 
metal from
crucible 
to
 
sprue.
 
Helps maintaining the required rate 
of 
liquid metal
flow.
 
Reduces turbulence 
at 
the sprue
 
entrance.
 
Helps separating 
dross, 
slag etc., 
from metal before it
enters 
the
 
sprue.
 
If the pouring basins are 
made
 
large,
Dross 
and slag 
formation will 
tend 
to float 
on 
the surface
of the 
metal 
and 
may 
be stopped 
from 
entering 
the sprue 
and
hence the
 
mould.
They 
may 
be 
filled quickly 
without 
overflowing 
and 
may
act 
as a 
reservoir of liquid metal 
to 
compensate metal
shrinkage or
 
contraction.
 
SPRU
E
 
A
 
sp
r
ue
 
f
e
eds
 
m
et
a
l
 
to
 
r
u
n
n
er
 
which
 
in
 
tu
r
n
 
re
a
ches
the 
casting 
through
 
gates.
 
A 
sprue 
is tapered with its bigger end 
at 
top 
to 
receive
the 
liquid 
metal. 
The 
smaller end is connected to
runner.
 
G
A
T
E
S
 
A gate is a channel which connects runner with the
mould cavity and 
through 
which molten 
metal 
flows
to fill the 
mould
 
cavity.
A small gate 
is 
used 
for 
a casting 
which 
solidifies
slowly and vice
 
versa.
A gate 
should 
not have sharp edges 
as 
they 
may 
break
during 
pouring and sand 
pieces 
thus 
may 
be 
carried
with 
the 
molten metal in 
the 
mould
 
cavity.
Types
Top
 gate
Bottom
 
gate
Parting 
line 
side
 
gate
 
T
OP
 
GATE
 
A 
top 
gate is 
sometimes 
also called 
as 
Drop 
gate
because the molten 
metal 
just 
drops 
on the 
sand 
in the
bottom of the
 
mould.
 
Generation 
of favourable 
temperature gradients to
enable 
directional solidification from the casting
towards the 
gate which serves 
as 
a riser
 
too.
 
Disadvantages
 
The 
dropping 
liquid 
metal 
stream 
erodes the mould
surface.
 
 
There is a lot of
 
turbulence.
 
B
OTTOM
 
GATES
 
A bottom gate is 
made 
in 
the 
drag 
portion of the
mould.
 
In 
a bottom gate 
the 
liquid metal fills rapidly the
bottom portion 
of 
the mould cavity and rises steadily
and gently up 
the 
mould
 
walls.
 
As 
comparison to 
top 
gate, bottom gate involves  little
turbulence 
and sand
 
erosion.
 
Bottom 
gate produces 
good 
casting
 
surfaces.
 
D
ISADVANTAGES
 
In 
bottom gates, liquid metal enters 
the 
mould cavity
at 
the 
bottom. 
If 
freezing takes place 
at 
the 
bottom, it
could 
choke 
off 
the 
metal 
flow 
before 
the 
mould is
full.
 
A 
bottom 
gate creates 
an 
unfavourable temperature
gradient 
and 
makes it 
difficult 
to 
achieve 
directional
solidification.
 
PARTING 
LINE SIDE
 
GATE
 
Middle or side or 
parting gating systems combine 
the
characteristics 
of top 
and bottom gating
 
systems.
 
In 
this technique gate is provided along 
the 
parting
line 
such that some portion 
of the 
mould cavity 
will
be 
below 
the 
parting line and some portion will 
be
above 
the 
parting
 
line.
 
The cavity below 
the 
parting line will 
be 
filled 
by
assuming 
top 
gating and the cavity above 
the 
parting
line 
will 
be 
filled 
by 
assuming bottom
 
gating.
 
DESIGN 
OF 
GATING
SYSTEM
 
To 
fill the mould cavity without breaking the 
flow 
of
liquid 
metal and without using very high pouring
temperatures.
To 
avoid erosion 
of 
mould
 
cavity.
To 
minimize turbulence and 
dross
 
formation.
T
o
 
p
r
event
 
aspi
r
a
t
ion
 
o
f
 
air
 
o
r
 
m
o
u
ld
 
g
a
ses
 
in
 
the
liquid 
metal
 
stream.
T
o
 
o
b
tain
 
favo
u
rable
 
te
m
perature
 
g
r
adien
t
s
 
to
promote directional
 
solidification.
 
D
EFECTS
 
OCCURRING
 
DUE
 
TO
 
IMPROPER
DESIGN
 
OF
 
GATING
 
SYSTEM
 
Oxidation 
of
 
metal
Cold
 
shuts
Mould
 
erosion
Shrinkages
Porosity
Misruns
Penetration 
of 
liquid 
metal 
into 
mould
 
walls.
 
R
EYNOLD
S
 
NUMBER
(R
E
)
 
Re
 
 
  
Vd
 
density
 
velocity
 
diameter
viscosity
 
C
RITICAL
 
R
EYNOLD
S
NUMBER
 
Re 
<
 
2,000
viscosity dominated, 
laminar
 
flow
 
Re 
>
 4,000
inertia 
dominated, 
turbulent
 
flow
 
Controlled through 
gate and runner
 
design
 
M
ETAL
 
FLOW
 
RATE
 
AND
 
VELOCITY
CALCULATIONS
 
S
t
u
d
i
e
s
 
of
 
gating
 
system
 
have
 
been
 
based
 
upon
 
t
wo
laws 
of fluid 
dynamics.
Law 
of
 
continuity
Q = 
A
1
V
1 
=
 
A
2
V
2
Q = volume rate 
of
 
flow
A = cross sectional area 
of flow
 
passage
V = linear velocity 
of
 
flow
 
P 
=
 
pressure
=
 
density
v =
 
velocity
 
h = 
height 
above the datum
 
plane
 
B
ERNOULLI
S
E
QUATION
 
Used 
to 
calculate 
flow
 
velocities
Assumptions: steady state, incompressible,
inviscid  
Flow
 
P
1
/
g 
+ 
V
1 
/ 
2g 
+ 
h
1 
= 
P
2
/
g 
+ 
V
2 
/ 
2g 
+
 
h
2
2
 
2
 
D
ESIGN
 
CRITERIA
 
FOR
 
POURING
 
BASIN
 
The
 
p
o
u
r
i
ng
 
basin
 
sho
u
l
d
 
b
e
 
designed
 
such
 
t
h
a
t
 
t
he
proper 
uniform flow 
system is rapidly
 
established.
This 
can 
be 
achieved
 
by-
 
Use of 
strainer
 
core
Use 
of 
DAM 
to 
make 
steady
 
flow
Use 
of sprue
 plug
 
It should 
be 
easy and convenient to 
fill pouring
 
basin.
 
D
ESIGN
 
OF
 
SPRUE
 
As 
the 
liquid metal passes 
down 
the sprue it 
loses its
pressure 
head 
but 
gains
 
velocity.
 
To 
reduce turbulence and promote Laminar 
Flow,
from the 
Pouring 
Basin, 
the flow 
begins a near
vertical incline 
that 
is acted 
upon by 
gravity and with
an 
accelerative gravity
 
force
 
hc
1
2
3
 
1 = free surface of
 
metal
2 
= 
spue
 
top
3 
= sprue
 
bottom
 
pouring
 
basin
sprue
 
h
t
 
Assuming
e
nti
re
 
m
o
u
ld
 
i
s
 
a
t
 
a
t
m
o
s
p
h
e
r
i
c
 
p
r
e
s
s
u
re
 
(
n
o
point 
below
 
atmospheric)
m
e
ta
l
 
i
n
 
t
he
 
po
u
r
i
n
g
 
b
a
s
i
n
 
i
s
 
a
t
 
ze
ro
 
ve
lo
c
ity
(reservoir 
assumption)
 
2
g
h
t
 
h
t
 
A
2
 
 
V
3
 
 
 
Mass 
flow 
rate = 
 A V =
 
constant
 
A
P
P
L
Y
ING
 
CON
T
I
N
U
I
TY
 
EQUATION
 
BETWE
E
N
 
P
O
INT
 
2
 
AN
D
 
3  
WE
GET
-
 
2
 
t
 
A
3
 
V
2
 
2
g
h
c
 
h
c
h
 
h
c
 
 
A
 
2
 
 
 
 
 
A
3
 
 
Actual shape of 
sprue 
is
 
Parabola
But in order to avoid manufacturing difficulty we use
tapered cylinder
 
shape
.
 
Tapered 
sprue 
reduces 
the rate of flow 
at 
which 
the
liquid 
metal 
enters 
the 
mould cavity and hence mould
erosion is
 
reduced.
The area 
at 
the sprue 
exit
 
controls-
Flow 
rate of 
liquid 
metal 
into 
mould
 
cavity
Velocity 
of liquid
 
metal
Pouring
 time
 
Ch
o
ke
 
is
 
t
h
at
 
part
 
o
f
 
t
h
e
 
ga
ti
ng
 
system
 
wh
i
ch
 
has
the 
smallest cross section
 
area.
In a 
free 
gating 
system 
sprue 
serves 
as
 
choke.
 
This
 
redu
c
es
 
m
o
u
ld
 
erosi
o
n
 
a
nd
 
tu
r
bul
e
nce
 
bec
a
use
velocity 
of liquid 
metal is
 
less.
This system causes air aspiration
 
effect.
 
In 
a 
choked system, 
gate serves 
as 
the
 
choke.
This creates a 
pressurized
 
system.
Due
 
to
 
high
 
metal
 
velocity
 
and
 
turbulence,
 
this
 
s
y
stem
 
exper
i
enc
e
s
 
o
x
i
d
a
tion
 
and
 
ero
s
ion
 
in
 
m
o
u
l
d
cavity.
The area 
at 
the 
sprue exit which if is 
the 
least
 
is
known 
as 
choke area and 
can 
be calculated from the
following
 
relation-
 
A
 
W
 
c
.
dt
 
2
gH
 
C
 
 
C
A
 
is choke
 
area
W is the weight of
 
casting
C is nozzle
 
coefficient
d is density 
of liquid
 
metal
t is 
pouring
 
time
H 
effective 
liquid 
metal
 
head
 
P
OURING
 
TIME
 
erosion,
 
rough
 
Hi
g
h
 
p
o
uring
 
rates
 
le
a
ds
 
to
 
m
o
u
ld
surface, excessive shrinkages
 
etc.
 
Low 
pouring 
rate 
may 
not permit the complete filling of
the 
mould cavity in 
time 
if 
the 
molten metal 
freezes 
fast
and thus defects like cold 
shuts 
may
 
develop.
 
It 
is very necessary to 
know 
optimum pouring rate 
or
pouring 
time 
for 
metals to 
be 
cast. Optimum 
pouring 
rate
a function 
of 
casting shape and
 
size.
 
Pouring 
time 
for 
brass 
or
 
bronze
V
aries
 
f
r
om
 
1
5
 
secon
d
s
 
t
o
 
4
5
 
seconds
 
m
a
y
 
b
e
 
u
s
e
d
for casting weighing less than 150
 
kg.
Pouring 
time 
for 
steel
 
casting
Steel
 
has
 
a
 
high
 
freezing
 
range
 
as
 
compared
 
to
 
other
 
c
a
st
 
all
o
ys,
 
it
freezing.
Pouring 
time
 
=
 
is
 
p
o
u
r
ed
 
r
a
pidly
 
t
o
 
avoid
 
e
a
rly
 
seconds
 
W is 
weight 
of casting in
 
lbs
K 
is fluidity
 
factor
 
Pouring 
time 
for 
gray cast iron
 
casting
casting weighing more than 1000
 
lbs.
 
 
Casting weighing less than 1000
 
lbs
 
 
W is weight of casting in
 
lbs
T is average section thickness in
 
inches
K 
is fluidity
 
factor
 
T
 
 
3 
w 
sec
 
onds
 
K 
0.95
 
 
 
0.85
3
 
T
 
 
w 
sec
 
onds
 
K 
0.95
 
 
 
0.853
 
Pouring time of light metal
 
alloys
Unlike steel, Al and Mg alloys are poured 
at 
a slow
rate, this is necessary to avoid turbulence, aspiration
and
 
drossing.
 
DESIGN OF 
RUNNER
 
AND 
 
GATES
 
In 
a good 
runner 
and gate
 
design-
Abrupt 
changes in section and sharp corners which
create turbulence 
and 
gas entrapment 
should be
avoided.
 
A suitable relationship must exist between 
the 
cross-
sectional 
area 
of sprue, runner 
and in
 
gates.
 
GATING
 
RATIO
 
Gating
 
ratio= a:b:c
 
where,
a= 
cross-sectional area of
 
sprue
b= 
cross-sectional area 
of
 
runner
c= 
total cross-sectional area 
of
 
ingates.
Gating ratio reveals-
whether 
the 
total cross- section decreases towards 
the
mould 
cavity. 
This 
provides 
a 
choke 
effect 
which
pressurizes 
the liquid 
metal in 
the
 
system.
Whether 
the 
total 
cross-sectional area increases 
so
that 
the 
passages 
remain 
incompletely 
filled. 
It is 
an
unpressurized
 
system
.
 
Ideally, 
in a system, pressure should 
be 
just enough to
avoid 
aspiration and keep to 
all 
feeding channels full
of liquid
 
metal.
 
Gating ratio and positions 
of 
ingates should 
be 
such
that the 
liquid metal fills 
the 
mould cavity just rapidly
to-
 
Avoid 
misruns 
and 
coldshuts in thin sectioned
castings.
 
Reduce turbulence and mould erosion 
in 
casting 
of
thicker
 
casting.
 
The 
maximum 
liquid metal tends to 
flow 
through the
farthest
 
ingate.
 
For 
a gating ratio 1:2:4, 
66% of 
liquid metal enters
through 
gate 
no. 
2 and only 34% does so through 
gate
no.
 
1.
 
Total 
ingate area is reduced 
by 
making gates farthest
from sprue of 
smaller cross-section 
so 
that 
less
volume of 
metal 
flows through 
them 
and 
makes a
uniform distribution of 
metal 
at 
all
 
ingates.
 
Besides with reduced total ingate area, 
still 
more
satisfactory result 
may 
be obtained if runner beyond
each 
ingate is 
reduced 
in cross section 
to 
balance 
the
flow 
in all parts 
of the 
system and to equalise further
velocity 
and
 
pressure.
 
S
TREAMLINING
 
THE
 
GATING
 
SYSTEM
 
Streamlining
 
includes-
 
Removing 
sharp 
corners or 
junction by giving 
a
generous
 
radius.
 
Tapering 
the sprue.
 
Providing radius 
at 
sprue 
entrance and
 
exit.
 
ADVANTAGES 
OF
 
STREAMLINING
 
Metal 
turbulence 
is
 
reduced.
 
Air 
aspiration 
is
 
avoided.
 
Mould 
erosion 
and 
dross 
are
 
minimized.
 
Sound 
and 
clean 
casting 
are
 
obtained.
 
P
OURING
 E
QUIPMENTS
 
Ladles are the commonly used equipment for
pouring the molten form the furnace. After
reading this article you will learn about the five
main types of pouring ladles.
The types are: 1. Hand Ladle 2. Shank or Bull
Ladle 3. Tea Pot Ladle 4. Bottom-Poured Ladle 5.
Monorail or Trolly Ladle.
 
46
 
 
Type # 1. Hand Ladle:
It is a bucket with removable, lever arm and
handle shank. It is used when the quantity of
molten metal is small. It can be carried by a
single person. Its carrying capacity varies from
10 to 20 kg. Fig. 4.7 (a).
Type # 2. Shank or Bull Ladle:
A shank or bull ladle is carried by two persons
and used for medium capacity of molten metal.
Its carrying capacity varies from 30 to 150 kg.
Fig. 4.7 (b).
 
47
 
 
Type # 3. Tea Pot Ladle:
Tea pot ladle is used for small and medium-sized
mould. Tea pot ladle allows the molten metal to
be taken out from the bottom opening provided.
The bottom opening is advantageous as it does
not disturb the slag floats on top. Fig. 4.7 (c).
 
48
 
 
Type # 4. Bottom-Poured Ladle:
Bottom poured ladle is used for top-run or direct-pour
into the mould. The molten metal is poured through
the bottom hole, which is operated by a graphite
stopper and lever. Slag, being lighter, floats at the top
of the molten metal and pure metal is poured into the
mould. Therefore, it is also known as self-cleaning
ladle. Fig. 4.7 (d).
Type # 5. Monorail or Trolly Ladle:
The molten metal is carried in a trolly. The trolly is
mounted on the mono­rail for easy movement to the
pouring site. The molten metal is poured through a
lever provided with crucible. A hand wheel is also
provided for raising and lowering the crucible. Fig.
4.7 (e).
 
49
 
 
50
 
A 
POOR
 
RUNNING
 & 
GATING
 
SYSTEM
:
 
A 
SATISFACTORY
 
RUNNING
 & 
GATING
SYSTEM
:
 
Design of Gating System:
 
R
I
S
E
R
I
N
G
 
O
F
 
C
A
S
T
I
N
G
S
 
A 
riser is 
a 
hole 
cut 
or 
moulded in 
the cope to
permit 
the molten metal to 
rise above 
the 
highest
point in 
the 
casting. 
The 
riser 
serves a 
number 
of
useful 
purposes. It enables 
the pourer to see the
metal  as 
it 
falls 
into 
the 
mould cavity. 
If the metal
does not appear 
in 
the 
riser, it signifies that either 
the
metal 
is  
insufficient 
to 
fill 
the 
mould 
cavity or there 
is
some obstruction 
to the 
metal 
flow 
between 
the sprue
and 
 
riser.
 
54
 
 
The
 
riser
 
facilitates
 
ejection
 
of
 
the
 
steam,
 
gas,
 
and
 
air
 
from
 
the
mould
 
cavity
 
as
 
the
 
mould
 
is
 
filled with 
the molten metal. 
Most
important, 
the 
riser serves 
as 
a feeder to feed the molten metal
into the main  casting 
to 
compensate for its shrinkage.
 
The
 
use
 
of
 
several
 
risers
 
may
 
be
 
necessary
 
in
 
the
 
case
 
of
 
an
intricate
 
or
 
large
 
casting
 
with
 
thin 
sections.
The 
main requisites 
of an 
effective 
riser are the
 
following:
i.
It 
must have sufficient volume 
as 
it should 
be the 
last part 
of 
the
casting 
to
 
freeze.
ii.
It 
must 
completely 
cover 
the 
sectional thickness that requires
feeding.
iii.
The 
fluidity 
of 
the molten 
metal 
must be 
adequately maintained
so that the metal can 
penetrate 
the  
portions 
of 
the 
mould cavity
freezing towards the
 
end.
iv.
It 
should be 
so designed that 
it establishes and effects
temperature gradients 
within 
the 
castings 
so  that the 
latter
solidifies directionally towards 
the
 
riser.
 
55
 
R
O
L
E
 
O
F
 
R
I
S
E
R
 
I
N
 
S
A
N
D
 
C
A
S
T
I
N
G
 
Metals and their alloys 
shrink 
as 
they cool 
or solidify and
hence may 
create 
a 
partial vacuum 
within  
the 
casting which
leads 
to 
casting defect known 
as 
shrinkage 
or 
void. 
The 
primary
function 
of 
riser 
as  
attached 
with 
the 
mould is 
to feed molten
metal to 
accommodate shrinkage occurring during solidification
of 
the
 
casting.
As shrinkage 
is 
very 
common 
casting defect in casting 
and
hence 
it should 
be 
avoided 
by 
allowing  
molten 
metal 
to 
rise in
riser after filling 
the 
mould cavity 
completely and supplying the
molten 
metal to  further 
feed the 
void 
occurred 
during
solidification 
of 
the 
casting 
because 
of
 
shrinkage.
 
56
 
 
 
Riser also permits 
the 
escape 
of 
evolved air 
and
mould 
gases as the 
mould cavity is being filled  with
the molten
 
metal.
It 
also indicates 
to 
the 
foundry 
man 
whether mould
cavity has been 
filled 
completely or 
not. The  suitable
design 
of 
riser also helps 
to 
promote 
the 
directional
solidification and 
hence 
helps in production 
of
desired 
sound
 
casting.
 
57
 
C
O
N
S
I
D
E
R
A
T
I
O
N
S
 
F
O
R
 
D
E
S
I
G
N
I
N
G
R
I
S
E
R
 
While designing risers the following considerations must
always be taken into account.
(A)
Freezing time
1.
For producing sound casting, the molten metal must be fed
to the mould till it solidifies completely.  This can be achieved
when molten metal in riser should freeze at slower rate than
the casting.
2.
Freezing time of molten metal should be more for risers than
casting. The quantative risering analysis  developed by Caine
and others can be followed while designing risers.
 
58
 
F
E
E
D
I
N
G
 
R
A
N
G
E
 
1.
When large castings are produced in complicated
size, then more than one riser are employed to  feed
molten metal depending upon the effective freezing
range of each riser.
2.
Casting should 
be 
divided into divided into different
zones 
so 
that 
each 
zone 
can be feed by a  separate
riser.
3.
Risers should 
be 
attached 
to 
that heavy 
section 
which
generally solidifies 
last 
in 
the
 
casting.
4.
Riser should maintain proper temperature gradients 
for
continuous feeding throughout freezing 
or  
solidifying.
 
59
 
F
E
E
D
 
V
O
L
U
M
E
 
C
A
P
A
C
I
T
Y
 
1.
Riser should have sufficient volume to feed the mould cavity till
the solidification of the entire casting  so as to compensate the
volume shrinkage or contraction of the solidifying metal.
2.
The metal is always kept in molten state at all the times in risers
during freezing of casting. This  can be achieved by using
exothermic compounds and electric arc feeding arrangement.
Thus it  results for small riser size and high casting yield.
3.
It is very important to note that volume feed capacity riser
should be based upon freezing time and  freezing demand.
Riser system is designed using full considerations on the
shape, size and the  position or location of the riser in the
mould.
 
60
 
E
F
F
E
C
T
 
O
F
 
R
I
S
E
R
 
Riser size affects on heat loss from 
top 
at 
open risers. 
Top 
risers are
expressed 
as 
a 
percentage
 
of  
total 
heat lost from 
the 
rises during
solidification. Risers 
are 
generally 
kept 
cylindrical. 
Larger the 
riser,  greater
is 
the 
percentage 
of 
heat 
that flows 
out 
of
 
top.
 
Shape
 
of
 
riser
 
may
 
be
 
cylindrical
 
or
 
cubical
 
or
 
of 
cuboids
 
kind.
 
If
 
shape
is
 
cylindrical
 
i.e.
 4
" 
high
 
and  4
" 
dia, insulated 
so that heat can pass 
only into
the circumferential 
sand 
walls, 
with 
a 
constant 
K 
value
 
of min./sq.ft.
Chvorinov’s
 
rule
 
may
 
be
 
used
 
to
 
calculate
 
the
 
freezing
 
time
 
for
 
cylinder
 
as
13.7 
min. 
The 
freezing time 
of 
a 
4
" 
steel 
cube 
of 
same sand 
is 
6.1 
minutes
and the 
freezing time 
of 
a 
2
"
,  
8
" 
and 
8
" 
rectangular block is also 
6.1
 
min.
 
Since 
the 
solidification time as calculated 
of 
the 
cylinder is nearly twice
as long 
as 
that 
of 
either the  block
 
of
 
the
 
cube.
 
Hence
 
cylindrical
 
shape
 
is
always
 
better.
 
Insulation
 
and
 
shielding
 
of
 
molten
 
metal
 
in
 
riser  also plays 
a
good 
role 
for 
getting sound
 
casting
 
61
 
T
Y
P
E
S
 
O
F
 
R
I
S
E
R
S
 
Risers
 
may
 
be
 
classified
 
as
 
open
 
risers
 
and
 
blind
risers
.
 
In
 
the
 
open
 
riser,
 
the
 
upper
 
surface
 
is
 
open
to
 
the
 
atmosphere
 
and
 
the
 
riser
 
is
 
usually
 
placed
 on
the
 
top
 
of
 
the
 
casting
 
or
 
at
 
the
 
parting
 
plane.
 
The
open  riser seldom extends downwards into 
the
drag, i.e., below 
the 
parting plane. 
This 
riser,
therefore, derives  feeding 
pressure 
from the
atmosphere and from 
the 
force 
of 
gravity 
on the
metal contained 
in 
the 
riser. 
In  case a 
certain
thickness 
of 
metal solidifies in the upper part 
of 
the
riser, atmospheric 
pressure no 
longer  
remains
effective, rendering 
metal flow from the 
riser 
to the
casting
 
difficult.
 
62
 
 
The
 
blind
 
riser,
 
on
 
the
 
other
 
hand,
 
is
 
surrounded
 
by
moulding
 
sand
 
on
 
all
 
sides
 
and
 
is
 
in
 
the
 
form 
of
 
a
 
rounded
 
cavity
in
 
the
 
mould
 
placed
 
at
 
the
 
side
 
or
 
top
 
of
 
the
 
casting.
 
It
 
may
 
be
located
 
either
 
in
 
the
 
cope  or 
in 
the 
drag. Since this riser is
closed 
from 
all sides, atmospheric pressure is 
completely shut
out. 
The  pressure due to the force 
of 
gravity is also reduced 
due
to the formation 
of 
vacuum 
within 
its
 
body.
In 
some 
of 
the 
improved designs, 
a 
permeable dry 
sand
core, 
fitted at 
the 
top 
of 
the blind riser,  extends 
up 
through 
the
cope 
to the 
atmosphere. Due 
to 
its permeable nature, air is able
to 
enter the riser  
and 
exert 
some 
pressure. 
There 
is also less
chilling 
effect, due to the use 
of 
dry 
sand 
core, and 
the
solidification 
of 
the 
riser is slowed down, 
thus 
making 
it 
more
effective.
 
63
 
 
Sometimes, artificial 
pressure 
is created in blind
risers 
by 
putting some explosive 
substance 
in the
riser
 
cavity.
 
When
 
the
 
substance
 
comes
 
in
 
contact
with
 
the
 
molten
 
metal,
 
it
 
explodes,
 
creating
 
high
pressure  within 
the
 
riser.
 
64
T
YPES
 
OF
 R
ISERS
:
 
D
I
R
E
C
T
I
O
N
A
L
 
S
O
L
I
D
I
F
I
C
A
T
I
O
N
 
Directional
 
solidification
 
is
 
the
 
solidification
 
of
molten
 
metal
 
from
 
the
 
sprue
 
to
 
the
 
mould
 
cavity
 
and
then  
to the 
riser 
to produce a 
casting which 
is free
from voids 
and 
internal
 
cavities.
 
As 
the molten metal 
cools in 
the 
mould 
and
solidifies, 
it 
contracts 
in volume. 
The 
contraction 
of
the  metal takes 
place in three
 
stages:
(i)
Liquid contraction;
(ii)
Solidification contraction;
 
and
(iii)
Solid contraction.
 
66
 
 
Liquid
 
contraction
 
occurs
 
when
 
the
 
molten
 
metal
cools
 
from
 
the
 
temperature
 
at
 
which
 
it
 
is
 
poured 
to
the
 
temperature
 
at
 
which
 
solidification
 
commences.
Solidification
 
contraction
 
takes
 
place
 
during
 
the
 
time
the 
metal changes 
from the 
liquid state 
to the 
solid,
e.g., when 
the 
metal loses its latent heat. Solid
contraction spans 
the 
period when 
the 
solidified metal
cools from freezing temperature 
to 
room
temperature.
 
67
 
 
Only 
the 
first two of these shrinkages are
considered 
for 
risering purposes, since 
the 
third is
accounted  
for by the 
patternmaker's contraction
allowance. Of 
the 
first two types, liquid shrinkage 
is
generally  negligible but solidification 
contraction 
is
substantial 
and should 
therefore 
be
 
considered.
 
68
 
 
Since
 
all
 
the
 
parts
 
of
 
the
 
casting
 
do
 
not
 
cool
 
at
 
the
 
same
rate,
 
owing
 
to
 
varying
 
sections
 
and
 
differing 
rates 
of 
heat
loss 
to 
adjoining mould 
walls, 
some parts tend 
to 
solidify
more 
quickly 
than 
others. 
This  
contraction phenomenon
causes 
voids 
and 
cavities 
in 
certain regions 
of 
the
casting.
These 
voids must 
be
 
filled 
up 
with 
liquid 
metal 
from 
the
portion 
of 
the 
casting 
that 
is still liquid 
and the
solidification should continue progressively 
from 
the
thinnest part, which 
solidifies, 
first, towards the risers,
which should be 
the 
last to solidify.
If 
the 
solidification 
takes 
place in this manner, 
the
casting 
will 
be 
sound  
with neither voids nor internal
shrinkage. 
This 
process is known 
as 
directional
solidification, 
and 
ensuring  its 
progress 
should 
be a
constant 
endeavor 
for 
the production 
of 
sound
 
castings.
 
69
 
 
In
 
actual
 
practice,
 
however,
 
it
 
may
 
not
 
always
 
be
 
easy
 
to
 
fully
achieve
 
directional
 
solidification
 
owing  
to
 
the
 
shape
 
and
design
 
of
 
the
 
casting,
 
the
 
type
 
of
 
casting
 
process
 
used,
 
and
such
 
other
 
factors.
 
In
 
general,  directional solidification can 
be
controlled
 
by
Proper 
design 
and 
positioning 
of 
the 
gating
system 
and 
risers  Inserting insulating sleeves 
for
risers
The use 
of 
padding 
to increase the 
thickness 
of 
certain sections 
of
the
 
casting
Adding exothermic material in 
the 
risers 
or 
in 
the facing sand 
around
certain portions 
of 
the  
castings
Employing chills in 
the M
oulds
Providing blind
 
risers
 
70
 
D
E
S
I
G
N
 
A
N
D
 
P
O
S
I
T
I
O
N
I
N
G
 
O
F
 
R
I
S
E
R
S
 
Riser 
Shape 
and 
Size
Riser 
Location
Types 
of 
Risers
Riserless Design
Use 
of 
Padding
Use 
of 
Exothermic Materials
Use 
of 
Chills
 
71
 
Chills
Introduction
A chill is an object used to promote solidification in a specific portion of a metal casting
mold.
Chill blocks are inserted into the mold to enhance the feeding distance by creating a steeper
temperature gradient.  The chill surface in contact with the casting must be clean and dry.
Chills can be used with a thin refractory coating or carbon black.  Cast iron or steel chills, for
all practical purposes, are equally effective.  Water-cooled copper chills are more effective
than uncooled cast iron or graphite. Graphite chills may deteriorate with use.
 
Classification of Chills
 
Internal chills
 
Internal chills are pieces of metal that are placed inside
the molding cavity. When the cavity is filled, part of the
chill will melt and ultimately become part of the
casting, thus the chill must be the same material as the
casting. Internal chills will absorb both heat capacity
and heat of fusion energy.
Internal chills are placed internally at locations in the
mold cavity that can't be reached with external chills.
Internal chill use is more problematic than external
chills. In external chills, the makeup isn't as critical
because they are outside the cavity; in internal chills the
metal used must be compatible with the metal being
poured.
In addition, the chill must have a melting temperature
nearly equal to that of the metal being poured.
Sometimes
, internal chills do not fuse completely with
the casting, thus establishing points of weakness, such
as lack of pressure tightness and radiographic
unsoundness.
Because internal chills will be completely surrounded
by metal, it is critical that they be clean. Gas created
from unclean internal chills can't readily escape.
 
External chills
 
External chills are masses of material that have a high
heat capacity and thermal conductivity. They are
placed on the edge of the molding cavity, and
effectively become part of the wall of the molding
cavity. This type of chill can be used to increase the
feeding distance of a riser or reduce the number of
risers required.
External chills are shapes usually made of steel, iron,
graphite, chromite or copper. They 
are p
laced where
hot spots or slow freezing rates may occur, these chills
are normally rammed up with the pattern and become
part of the mold wall. They not only promote
directional solidification but also 
affect
 temperature
gradients that reduce the possibility of micro-porosity.
External chills are used effectively at junctions or other
portions of the casting that are difficult to feed with
risers.
Chill size is determined by the cooling requirement.
Generally, a chill's thickness shouldn't be less than that
of the metal section it is chilling. They are frequently
covered with a protective wash, silica flour or other
refractory material.
 
Riser
 
Design
Lesson
 
Objectives
In this chapter 
we 
shall discuss the 
following:
Solidification 
of
 
casting
Chvorinov 
rule
Functions 
of
 
riser
Types of
 
riser
Methods for riser
 
design
Learning
 
Activities
1.
Look 
up
K
e
ywords
2.
View
 
Slides
;
3.
Read
 
Notes,
4.
Listen
 
to
lecture
 
C
o
o
l
ing
 
characteristics,
 
Keywords:
 
Solidification
 
shrinkage,
Freezing
 
ratio,
 
modulus,
 
NRL
 
method
 
Solidification 
of
 
Casting
 
During
 
so
l
idi
f
icati
o
n
 
met
a
l
 
e
x
peri
e
n
ce
 
s
hri
n
ka
g
e
 
whi
c
h
results in void formation.
This
 
can
 
b
e
 
a
v
oi
d
ed
 
by
 
f
e
eding
 
h
ot
 
sp
o
t
 
during
solidification.
Riser 
are 
used to 
feed 
casting during
 
solidification.
 
Solidification of Iron & Carbon
Steels
 
Figure 10.5 (a) Solidification patterns 
for 
gray cast iron in a 
180-mm 
(7-in.) square casting. Note that
after 
11 
minutes 
of cooling, dendrites reach each 
other, 
but the casting is still 
mushy 
throughout. 
It 
takes
about two hours for this casting to solidify 
completely. 
(b) 
Solidification of carbon steels in sand and
chill 
(metal) molds. 
Note the difference in solidification patterns as the carbon content
 
increases.
 
What 
Are
 
Risers?
 
Risers 
are 
added reservoirs designed to feed liquid
metal to the solidifying casting as 
a means 
for
compensating for solidification
 
shrinkage.
Riser must solidify 
after
 
casting.
Riser should 
be 
located so that directional
solidification 
occurs 
from 
the 
extremities of mold
cavity back toward the
 
riser.
Thickest 
part of casting 
last to freeze, Riser should
feed 
directly to these
 
regions.
 
Why
 
Risers?
 
The shrinkage occurs 
in three
 
stages,
1.
When 
temperature of 
liquid 
metal drops 
from Pouring
to Freezing
 
temperature
2.
Wh
e
n
 
t
h
e
 
m
e
t
a
l
 
ch
anges
 
fr
o
m
 
liquid
 
t
o
 
s
o
lid
 
s
t
a
t
e,
and
3.
Wh
e
n
 
t
h
e
 
temper
at
ure
 
o
f
 
s
ol
i
d
 
p
h
ase
 
dr
o
ps
 
fr
o
m
freezing to room
 
temperature
 
The 
shrinkage for 
stage 
3 
is compensated 
by 
providing
shrinkage allowance 
on pattern, while the 
shrinkage
during 
stages 1 and 2 are 
compensated 
by 
providing
risers.
 
Riser 
Location &
 
Types
 
Solidification 
Time For
 
Casting
 
Solidification of 
casting occurs 
by 
loosing 
heat from the
surfaces 
and 
amount 
of heat is 
given by 
volume 
of
casting
 
.
Cooling
 
c
h
aracteris
t
ic
s
 
of
 
a
 
ca
s
tin
g
 
i
s
 
t
he
 
ra
t
i
o
 
of
surface
 area
 
to
 
volume
.
Higher 
the 
value of 
cooling 
characteristics 
faster 
is the
cooling 
of
 
casting.
Chvorinov
 
rule
 
state
 
that
 
solidification time 
is 
inversely
proportional to 
cooling
 
characteristics.
 
Where
 
Ts 
= 
Solidification
 
time
 
SA = 
Surface
 
area
 
V
 
= 
Volume 
of
 
casting
K = mould
 
constant
 
A cylindrical 
riser must 
be 
designed for 
a 
sand-casting
mold. 
The casting 
itself is 
a 
steel rectangular 
plate
with dimensions 7.5 
cm 
x12.5 
cm x 
2.0 cm. Previous
observations have indicated that 
the 
solidification
time for this casting is 1.6 min. The 
cylinder 
for 
the
riser will 
have a 
diameter-to-height ratio as 1.0.
Determine 
the dimensions of the 
riser 
so that 
its
solidification time is 2.0
 
min.
V
/
A
 
ra
t
i
o
 
=
 
(7
.5
 
x
 
1
2
.5
 
x
 
2
)
 
/
 
2(7
.
5
x
12
.
5
 
+
 
12
.
5
x
2
 
+
7.5x2) 
= 
187.5 
/ 
267.5 
=
 
0.7
 
Methods 
of 
Riser
 
Design
 
Following 
are 
the methods 
for 
riser
 
design:
 
1.
Caine’s
 
Method
2.
Modulus
 
Method
3.
NRL
 
Method
 
Caine’s
 
Method
 
Caine’s
 
equation
 
Where
X = Freezing 
ratio
Y = Riser 
volume 
/ Casting 
volume
A, b and c =
 
Constant
 
Freezing
 
ratio
+
 
Constant For Caine’s
 
Method
 
Values of constants are 
given 
in
 
table:
 
NRL
 
Method
 
NRL stand 
for 
Naval 
research
 
Laboratory.
NRL method is 
essentially 
a simplification of Caine’s
 
method.
In 
this 
method shape 
factor 
is used in place of 
freezing
 
ratio.
 
Shape
 
factor
=
 
NRL
 
Method
 
Ratio 
of 
riser volume 
to casting 
volume 
can be 
obtained
 
from
graph shown
 
below.
After 
obtaining 
riser volume riser 
diameter and height can 
be
obtained.
Use H/D = 1 
for Side riser 
and H/D 
=0.5 for Top
 
riser
 
Choke
 
Area
 
Choke
 
area
 
is
 
the
 
main
 
control
 
area
 
which
 
meters
 
the
 
metal
flow 
into mould
 
cavity.
No
r
mal
l
y
 
choke
 
a
re
a
 
ha
p
pens
 
t
o
 
b
e
 
at
 
t
h
e
 
bo
t
t
om
 
o
f
 
the
sprue so sprue should 
be 
designed
 
first.
Having
 
sprue
 
bottom
 
as
 
the
 
choke
 
area
 
help
 
in
 
establishing
proper 
flow 
in the mould easily and
 
early.
Choke area can 
be 
calculated 
by 
Bernoulli’s
 
equations
Q=
 
AV
W
 
=ρAV
Choke area A = 
W/
 
ρV
= 
W/
 
ρ√2gH
= 
W/ 
ρ t c
 
√2gH
 
Effective 
Sprue
 
Height
 
Ef
f
e
c
tiv
e
 
sprue
 
h
e
i
g
ht
 
H
,
 
o
f
 
a
 
m
o
uld
 
dep
e
nd
s
 
o
n
 
t
h
e
casting dimensions 
and 
type of gating
 
system.
It 
can be calculated using 
following
 
relations:
 
Where
h =Sprue
 
height
p = Height of 
mould 
cavity in
 
cope
c = 
Total 
height of 
mould
 
cavity
 
Values 
of h, 
P 
and c are 
shown 
in for various gating
 
system
 
Efficiency Coefficient For
Gating
 
Systems
 
Pouring
 
Time
 
Time required for filling 
a 
mould is pouring
 
time.
Too long pouring 
time 
Higher 
pouring
 
temperature
Too less pouring time – 
Turbulent 
flow 
& 
defective
casting.
It depends on 
casting material, 
complexity 
of casting
,
section 
thickness 
and 
casting
 
size
.
Pouring 
time is calculated by 
empirical formulas 
obtained
by 
experiments 
which 
differ from 
one 
material to
another and one 
casting to
 
other.
For non 
ferrous 
material, 
long pouring 
time would 
be
beneficial 
since 
they 
lose 
heat 
slowly 
and 
also 
tend 
to
form dross 
if metal is poured too
 
quickly.
 
Pouring
 
Time
 
Grey 
cast iron, 
mass 
less than 450
 
kg
 
Grey 
cast iron, 
mass 
greater than 450
 
kg
 
Steel
 
castings
undefined
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In Unit 4 of Foundry Technology, the focus is on Gating System, Risering System, and Quality Control in Foundry. The content covers components of the gating system, considerations for defect-free casting, proper design requirements, and factors affecting the functioning of the gating system. Key topics include gating design, risering practice, casting defects and remedies, and quality control tests. Understanding these concepts is essential for ensuring a successful and efficient foundry operation.


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  1. FOUNDRY TECHNOLOGY UNIT 4 1 -Monil Salot

  2. SYLLABUS UNIT 4 Gating System: Elements of gating system. Classification. Gating design considerations, Gating ratio. Pouring equipments. Risering System: Risering practice Functions of riser, Directional and progressive solidification. Centerline feeding resistance. Riser efficiency. Riser design considerations. Risering curves. Cain s, N.R.L. and Modulus methods, Feeding distance and feeding aids, Blind and atmospheric risers Quality Control in Foundry: Casting defects, their causes and remedies. Shop floor quality control tests such as composition control, Wedge test, fluidity, temperature measurement etc. 2

  3. GATINGSYSTEM The term gating system refers to all passageways through which the molten metal passes to enter the mould cavity. The gating system is composed of Pouring basin Sprue Runner Gates Risers

  4. Components of Gating System

  5. Any gating system designed should aim at providing a defect free casting. This can be achieved by considering following requirements. A gating system should avoid sudden or right angle changes in direction. A gating system should fill the mould cavity beforefreezing. The metal should flow smoothly into the mould without any turbulence. A turbulence metal flow tends to form dross in the mould. Unwanted materials such as slag, dross and other mould materials should not be allowed to enter the mould cavity. The metal entry into the mould cavity should be properly controlled in such a way that aspiration of the atmospheric air is prevented.

  6. A proper thermal gradient should be maintained so that the casting is cooled without any shrinkage cavities ordistortions. Metal flow should be maintained in such a way that no gating or mould erosion takes place. The gating system should ensure that enough molten metal reaches the mould cavity. It should be economical and easyto implement and remove after casting solidification.

  7. TH For proper functioning of the gating following factors need to be controlled. E Type of basin etc. Temperature/ Fluidity of molten metal. Rate of liquid metal pouring. Type and size of sprue. Type and size of runner. Size, number and location of gates connecting runner and casting. Position of mould during pouring and solidification. pouring equipment, such as ladles, pouring

  8. POURING BASINS

  9. A pouring basin makes it easier for the ladle or crucible operator to direct the flow of metal from crucible to sprue. Helps maintaining the required rate of liquid metal flow. Reduces turbulence at the sprue entrance. Helps separating dross, slag etc., from metal before it enters the sprue.

  10. If the pouring basins are made large, Dross and slag formation will tend to float on the surface of the metal and may be stopped from entering the sprue and hence the mould. They may be filled quickly without overflowing and may act as a reservoir of liquid metal to compensate metal shrinkage or contraction.

  11. SPRU E A sprue feeds metal to runner which in turn reaches the casting through gates. A sprue is tapered with its bigger end at top to receive the liquid metal. The smaller end is connected to runner.

  12. GA TES A gate is a channel which connects runner with the mould cavity and through which molten metal flows to fill the mould cavity. A small gate is used for a casting which solidifies slowly and vice versa. A gate should not have sharp edges as they may break during pouring and sand pieces thus may be carried with the molten metal in the mould cavity. Types Top gate Bottom gate Parting line side gate

  13. TOPGATE A top gate is sometimes also called as Drop gate because the molten metal just drops on the sand in the bottom of the mould. Generation of favourable temperature gradients to enable directional solidification from the casting towards the gate which serves as a riser too.

  14. Disadvantages The dropping liquid metal stream erodes the mould surface. There is a lot of turbulence.

  15. BOTTOM GATES A bottom gate is made in the drag portion of the mould. In a bottom gate the liquid metal fills rapidly the bottom portion of the mould cavity and rises steadily and gently up the mould walls. As comparison to top gate, bottom gate involves little turbulence and sand erosion. Bottom gate produces good casting surfaces.

  16. DISADVANTAGES In bottom gates, liquid metal enters the mould cavity at the bottom. If freezing takes place at the bottom, it could choke off the metal flow before the mould is full. A bottom gate creates an unfavourable temperature gradient and makes it difficult to achieve directional solidification.

  17. PARTING LINE SIDE GATE Middle or side or parting gating systems combine the characteristics of top and bottom gating systems. In this technique gate is provided along the parting line such that some portion of the mould cavity will be below the parting line and some portion will be above the parting line. The cavity below the parting line will be filled by assuming top gating and the cavity above the parting line will be filled by assuming bottom gating.

  18. DESIGN OF GATING SYSTEM To fill the mould cavity without breaking the flow of liquid metal and without using very high pouring temperatures. To avoid erosion of mould cavity. To minimize turbulence and dross formation. To prevent aspiration of air or mould gases in the liquid metal stream. To obtain favourable promote directional solidification. temperature gradients to

  19. DEFECTS OCCURRING DUE TO IMPROPER DESIGN OF GATING SYSTEM Oxidation of metal Cold shuts Mould erosion Shrinkages Porosity Misruns Penetration of liquid metal into mould walls.

  20. REYNOLDS NUMBER (RE) Re = Vd =(density) (velocity) (diameter) (viscosity)

  21. CRITICAL REYNOLDS NUMBER Re < 2,000 viscosity dominated, laminar flow Re > 4,000 inertia dominated, turbulent flow Controlled through gate and runner design

  22. METAL FLOW RATE AND VELOCITY CALCULATIONS Studies of gating system have been based upon two laws of fluid dynamics. Law of continuity Q = A1V1 =A2V2 Q = volume rate of flow A = cross sectional area of flowpassage V = linear velocity of flow

  23. BERNOULLIS EQUATION Used to calculate flow velocities Assumptions: steady state, incompressible, inviscid Flow P1/ g + V1 / 2g + h1 = P2/ g + V2 / 2g +h2 2 2 P =pressure = density v = velocity h = height above the datumplane

  24. DESIGN CRITERIA FOR POURING BASIN The pouring basin should be designed such that the proper uniform flow system is rapidly established. This can be achieved by- Use of strainer core Use of DAM to make steady flow Use of sprue plug It should be easy and convenient to fill pouring basin.

  25. DESIGN OF SPRUE As the liquid metal passes down the sprue it loses its pressure head but gains velocity. To reduce turbulence and promote Laminar Flow, from the Pouring Basin, the flow begins a near vertical incline that is acted upon by gravity and with an accelerative gravity force

  26. pouring basin sprue 1 hc 2 ht 1 = free surface of metal 2 = spue top 3 = sprue bottom 3 Assuming entire mould is at atmospheric pressure (no point below atmospheric) metal in the pouring basin is at zero velocity (reservoir assumption)

  27. Mass flow rate = A V =constant APPLYINGCONTINUITY BETWEEN POINT GET- EQUATION 2 AND 3 WE 2ght 2ghc ht hc A2=V3= A3 = V2 2 h hc A A3 t = 2 Actual shape of sprue is Parabola But in order to avoid manufacturing difficulty we use tapered cylinder shape.

  28. Tapered sprue reduces the rate of flow at which the liquid metal enters the mould cavity and hence mould erosion is reduced. The area at the sprue exit controls- Flow rate of liquid metal into mould cavity Velocity of liquid metal Pouring time Choke is that part of the gating system which has the smallest cross section area. In a free gating system sprue serves as choke.

  29. This reduces mould erosion and turbulence because velocity of liquid metal is less. This system causes air aspiration effect. In a choked system, gate serves as the choke. This creates a pressurized system. Due to high metal velocity and turbulence, this system experiences oxidation and erosion in mould cavity. The area at the sprue exit which if is the least is known as choke area and can be calculated from the following relation-

  30. W = C A c.dt 2gH CAis choke area W is the weight of casting C is nozzle coefficient d is density of liquid metal t is pouring time H effective liquid metal head

  31. POURING TIME High pouring rates leads to mould surface, excessive shrinkages etc. erosion, rough Low pouring rate may not permit the complete filling of the mould cavity in time if the molten metal freezes fast and thus defects like cold shuts may develop. It is very necessary to know optimum pouring rate or pouring time for metals to be cast. Optimum pouring rate a function of casting shape and size.

  32. Pouring time for brass or bronze V aries from 15 seconds to 45 seconds may be used for casting weighing less than 150 kg. Pouring time for steel casting Steel has a high freezing range as compared to other cast alloys, it freezing. Pouring time = seconds is poured rapidly to avoid early W is weight of casting inlbs K is fluidity factor

  33. Pouring time for gray cast iron casting casting weighing more than 1000 lbs. T 3 w seconds K 0.95+ 0.853 Casting weighing less than 1000 lbs T w seconds K 0.95+ 0.853 W is weight of casting in lbs T is average section thickness in inches K is fluidity factor

  34. Pouring time of light metal alloys Unlike steel, Al and Mg alloys are poured at a slow rate, this is necessary to avoid turbulence, aspiration and drossing.

  35. DESIGN OF RUNNERAND GATES In a good runner and gate design- Abrupt changes in section and sharp corners which create turbulence and gas entrapment should be avoided. A suitable relationship must exist between the cross- sectional area of sprue, runner and in gates.

  36. GATING RATIO Gating ratio= a:b:c where, a= cross-sectional area of sprue b= cross-sectional area of runner c= total cross-sectional area of ingates. Gating ratio reveals- whether the total cross- section decreases towards the mould cavity. This provides a choke effect which pressurizes the liquid metal in the system. Whether the total cross-sectional area increases so that the passages remain incompletely filled. It is an unpressurized system.

  37. S.N. Pressurized gating systems Unpressurized gating systems 1. Gating ratio may be of the order of 3: 2: 1 Gating ratio may be of the orderof 1: 3: 2 2. Air aspiration effect isminimum Air aspiration effect ismore 3. Volume flow of liquid from every ingate is almostequal. Volume flow of liquid from every ingate is different. 4. They are smaller in volume for a given flow rate Therefore the casting yield is higher. They are larger in volume because they involve large runners and gates as compared to pressurized system and thus the cast yield isreduced. of metal. 5. V elocity turbulence may occur at corners. is high, severe V elocity is low and turbulence is reduced.

  38. Ideally, in a system, pressure should be just enough to avoid aspiration and keep to all feeding channels full of liquid metal. Gating ratio and positions of ingates should be such that the liquid metal fills the mould cavity just rapidly to- Avoid misruns and coldshuts in thin sectioned castings. Reduce turbulence and mould erosion in casting of thicker casting.

  39. The maximum liquid metal tends to flow through the farthest ingate. For a gating ratio 1:2:4, 66% of liquid metal enters through gate no. 2 and only 34% does so through gate no. 1. Total ingate area is reduced by making gates farthest from sprue of smaller cross-section so that less volume of metal flows through them and makes a uniform distribution of metal at all ingates.

  40. Besides with reduced total ingate area, still more satisfactory result may be obtained if runner beyond each ingate is reduced in cross section to balance the flow in all parts of the system and to equalise further velocity and pressure.

  41. STREAMLINING THE GATING SYSTEM Streamlining includes- Removing sharp corners or junction by giving a generous radius. Tapering the sprue. Providing radius at sprue entrance and exit.

  42. ADVANTAGES OFSTREAMLINING Metal turbulence is reduced. Air aspiration is avoided. Mould erosion and dross are minimized. Sound and clean casting are obtained.

  43. POURING EQUIPMENTS Ladles are the commonly used equipment for pouring the molten form the furnace. After reading this article you will learn about the five main types of pouring ladles. The types are: 1. Hand Ladle 2. Shank or Bull Ladle 3. Tea Pot Ladle 4. Bottom-Poured Ladle 5. Monorail or Trolly Ladle. 46

  44. Type # 1. Hand Ladle: It is a bucket with removable, lever arm and handle shank. It is used when the quantity of molten metal is small. It can be carried by a single person. Its carrying capacity varies from 10 to 20 kg. Fig. 4.7 (a). Type # 2. Shank or Bull Ladle: A shank or bull ladle is carried by two persons and used for medium capacity of molten metal. Its carrying capacity varies from 30 to 150 kg. Fig. 4.7 (b). 47

  45. Type # 3. Tea Pot Ladle: Tea pot ladle is used for small and medium-sized mould. Tea pot ladle allows the molten metal to be taken out from the bottom opening provided. The bottom opening is advantageous as it does not disturb the slag floats on top. Fig. 4.7 (c). 48

  46. Type # 4. Bottom-Poured Ladle: Bottom poured ladle is used for top-run or direct-pour into the mould. The molten metal is poured through the bottom hole, which is operated by a graphite stopper and lever. Slag, being lighter, floats at the top of the molten metal and pure metal is poured into the mould. Therefore, it is also known as self-cleaning ladle. Fig. 4.7 (d). Type # 5. Monorail or Trolly Ladle: The molten metal is carried in a trolly. The trolly is mounted on the monorail for easy movement to the pouring site. The molten metal is poured through a lever provided with crucible. A hand wheel is also provided for raising and lowering the crucible. Fig. 4.7 (e). 49

  47. 50

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