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Sand Mold and Gating System

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Manufacturing Processes – Casting and Joining
Prof. Sounak Kumar Choudhury
Department of Mechanical Engineering
Indian Institute of Technology Kanpur
Lecture – 04
Hello and welcome back to the course on Manufacturing Processes - Casting and Joining. Let
me remind you that in our last discussion session we started discussing moulds and I showed
you one typical sand mould where we have the two parts of the flask.
(Refer Slide Time: 00:43)
If you lo at this diagram - this is the cross section of the typical sand mold we have the cope
which is the upper part and the drag which is the lower part as I said. here if you see the molten
metal is poured in the pouring basin which is also called the pouring cup and it flows through
the sprue and the gate to the pouring well. From here the molten metal runs through the runner
and it fills up the blind riser, which is a cavity, as well as the mold cavity which is also a space
for the casting.
Here we have the open riser and . there is a core. This core is used for making the internal
cavity or internal shape or the internal silhouette of the casting. this is the entire design of the
typical sand mold. This portion is filled up with the sand as you have seen in the video yesterday
or in our previous class.And another thing that it has is a vent. This vent is specially made so that the gases inside, after
the molten metal is poured, can escape. This is called the sprue and in our previous class I also
told you that in sprue design or in the gating system design, this design is little critical. We will
discuss it in details at a later stage. Here as for example, here what you can see is it is a taper
shapesprue.
There are different kind of shapes that can be found out from the design and those shapes are
particularly made to avoid the aspiration of the air, so that the air does not enter into the molten
metal. We will discuss at a later stage why aspiration happens.
(Refer Slide Time: 03:26)
This is the schematic of the sand mould. Here is the pouring cup. Molten metal flows through
the sprue to the well. The sprue is of a taper shape. Through the runner the molten metal enters
into the cavity. There is a core here that is used to make the internal hole in this case. This is
the core print and this is the mould cavity.
Here we have a riser which is a closed riser and the molten metal will run through the runner
to fill up this space and through the ingate it flows into the mould cavity to fill it up and to get
the casting finally. When the entire cavity is filled up including the riser, we will understand
from the level of the molten metal in the pouring cup that you have seen in the video in our
previous classes.Then the pouring stops and it is kept for the solidification. After the solidification the mould is
bren. This is the cope, one half of the flask, and the another half or the lower part is the drag.
Both of them will be bren and the sand mould is destroyed to take out the casting. Now, if you
see here, with the casting you will also have the riser and the entire gating system up to this.
This will also be solidified and it will be attached to the casting. So, in this place where it is
attached to the casting, it has to be sheared off , then you will have the actual casting, then of
course, the core has to be removed. And then what you will have is a casting with actually an
internal hole. This is the final shape of the casting that you can see. This is the core box where
the core can be made and in the core box special sand is used with molasses .
A big age of molasses, so that it can bind. Molasses acts as a binder and a special sand is used
for the core. Then the core is baked to get the sufficient strength and that core is placed inside
the cavity so that around the core when the metal material molten material is filled up, the
core can be taken out after the solidification and the casting can be made with an internal hole.
That is the basic purpose of the core that I already told you in my previous classes that for any
kind of internal shape we have to use the core in the sand mould casting .
(Refer Slide Time: 06:44)
The flask is the box that contains the molding aggregate, the entire thing is the flask this is the
box, in this box we have placed the sand, pattern, riser and so on that you have seen in the video clip. Cope is the top half of the flask. This is the cope and the lower half or the bottom
half of the flask is called the drag.
The core is inserted to produce the holes, that means the hollow parts here to get this hole we
have to get the core inserted in the mould cavity.
(Refer Slide Time: 07:32)
Riser is the extra void filled with metal. It supplies metal to mould cavity to compensate for
the shrinkage solidification. I discussed it earlier that riser is actually a reservoir of molten
metal. The molten metal will flow in and in the riser design we should make sure that the
molten metal inside the riser should solidify after the solidification of the molten metal inside
the mould cavity.
When the metal is solidified inside the mould cavity, there will be shrinkage, various kind of
shrinkages I already told you, and to compensate for those shrinkages we have to supply molten
metal and that molten metal should come from the riser. Therefore, riser is actually the
reservoir for the molten metal for compensating the shrinkage cavities and the other cavities in
the solidified casting. That is the basic purpose .
Now, the gating system is the network, the entire network, as I told you, is called the gating
system. This is the network of channels used to deliver the molten metal to the mould cavity. Molten metal will enter to the mould cavity from here through this well, runner , and the gate
here . All these elements will be included in the gating system, which are pouring cup, spruces,
runners, ingates. Ingates are here.
(Refer Slide Time: 09:42)
Now all those things have to be sheared off from the final casting, so that the casting could be
used. The green sand mold composition is the following: it has about 70% to 85% of sand.
Now, how the sand is selected is a different issue. The quality of the casting will depend greatly
on the type of the sand that you are using. Whether it is a coarse or it is a fine grain? How much
clay is there? How much water is there? All those things have to be very judiciously considered
and those things also we will discuss at a later stage.
Now, along with the 70% to 85% of sand, there is about 10 to 20% of clay ; then 3 to 6% of
water. There are additives like wood flour, dextrin, sea coal is about 1 to 6%.
Now, the clay is for binding as you understand and along with the water. Water cannot be less
than 3% , otherwise it will not bind, but it cannot be more than 6% ; because otherwise the
voids will be filled up by the water ; void is the space between the grains .
Shape and size of sand grains vary very widely. The bulk density of a sand mix is very low, if
the grains are of equal size with smooth and the round shape. Because you understand that, if
it is of a round shape, then when they are ideally in touch with each other, the distance from
the center of one grain to another grain is actually the diameter of the grain .So, overall we say that, bulk density of a sand mix is very low if the grains are of equal size
with smooth and the round shape. Inequal grain size will result in increased voids and the higher
permeability.
Increased voids mean, spaces will be more between the grains. And once the spaces will be
more between the grains, the permeability will be more; permeability is the possibility or
capability of the sand mould to allow the gases to escape, to come out.
Clay together with water acts as a bonding agent, imparting tensile and the shear strength to
the moulding sand. Without the clay and water together, we will not get the sufficient tensile
and the shear strength of the moulding sand or the overall mould . The organic additives burn
out at high temperatures making room for the moulding sand to expand and thus save the mould
from crumbling.
Now, I said in my one of my earlier lectures that, we have to be very careful about the material
of the flask. Because after the molten metal is poured, the mould will expand and the flask has
to give way, flask has to allow this expansion of the mould.
This is helped by these organic additives; they burn out at high temperature of the molten metal
as the molten metal goes in, that makes the room for the moulding sand to expand and thus
save the mould from crumbling. That is why those organic additives like wood flour, dextrin,
sea coal are used to get the flexibility of the sand mould.
(Refer Slide Time: 14:05)Properties of moulding sand: these are the five basic properties, which the ideal moulding sand
should have. First, it has to be strong, that is, the compressive strength should be higher.
Permeability is the gas flow rate through the specimen under a specified pressure difference
across it. When the pressure difference is there, the gas will be evolved. And when the gas will
come out, it has to be allowed to escape by the moulding sand. If the permeability is less in
case when the voids are small or the voids are occupied by the higher percentage of water for
example,
in that case, the gas will not be able to come out and it will actually create the porosity; it will
create the defects inside the casting. Third property is the deformation that is the change in
length of a standard specimen at the point of failure. Next is the flowability; that is the ability
of the sand to flow around and over the pattern when the mould is rammed. This is very
important that sand has to fill up the entire mould cavity wherever it is required.
That is why it is actually rammed. It is important for the moulding sand to have the ability to
flow or the flowability, so that it can fill up, flow around and over the pattern, so that it can
wrap the pattern around. And the fifth property of the moulding sand is the refractoriness; that
is the ability of the sand to remain solid as a function of temperature.
At a higher temperature, temperature is very high because of the molten metal as you
understand, the sand should not lose its strength, or any other property and it should remain
solid. That is the refractoriness. So, there is a resistance to heat, so that it can actually withstand
that much temperature.(Refer Slide Time: 16:35)
Let us see the curve of the tensile strength and the water percentage, that is the moisture. If you
remember I was just telling you that, the water percentage cannot be more than 6%; we said
that water percentage is from 3 to 6%. It cannot be less than 3%, so that it can bind properly.
Why not more than 6%? Here is the reason. Let us say this is the curve of the tensile strength
and this is the weight percentage of the moisture or the water content. As the water content is
increasing; let us say weight percentage is given in 2, 4, 6 up to 8 along the X-axis.
Initially when the water is added to the clay, the clay particles will actually expand. And they
will push the sand particles apart from each other, making the voids more and the permeability
will be more. Because as I said just now that, as the dimension or the size of the void increases,
then the permeability increases; that means the ability of the mould material to pass air, pass
the gas will be more.
So, the permeability actually increases and then it goes up to a certain point; let us say it is at
3% here and this experiment has been made with the 6% of clay let us say. So, it goes up to 3
weight percent of the moisture; after that if we keep on adding water, in that case actually the
strength goes down.
And strength goes down because of the fact that, the increased voids are filled up by the water.
In that case, those voids will not be effective for allowing the gas through them and the strength decreases. If you see here, initially when the water is mixed with the clay, the strength and
permeability increase.
Permeability increases because the grains or the particles of the clay are expanding and they
are pushing the clay, pushing the sand particles apart from each other. So, the density decreases,
permeability increases up to 3% let us say in this case. After that, the permeability starts
decreasing along with the strength because the voids are filled up with the wate. And as you
can see here the density increases, because the voids are filled up with the water.
So, if you read it here you can understand, this is for your note; it can be seen that the strength
increases to a maximum at about 3% of moisture, let us say here this is the 3% . With the
further increase in the percentage of water slowly out to 8 percent of moisture, the strength
decreases.
Increasing the strength from 0 to 3% reflects the taking on of water by the clay resulting in the
swelling of the clay particles, thereby pushing the sand particles apart. This I already explained
to you, why this density decreases and why the strength increases and the permeability
increases; because the sand particles are pushed by the expansion of the clay particles,
thereby pushing the sand particles apart, resulting in reduced density and increased
permeability. At about 3%, the clay becomes saturated with water. And beyond this point, the
water merely fills space in the void volume, resulting in an increase in density; density is
increased after that and both the permeability and the tensile strength are decreased.
So, this is the curve as shown in the figure. This is what happens with the strength, density and
the permeability as the water is being added to the clay in the mould material. When the
moulding sand is made, the mould is made, in that case first the clay powder is added and then
to that the water is added and this is what happens up to 3 weight percent of moisture.
After that of course, it is detrimental; it does not make any sense in adding more water, because
it actually reduces the properties of the sand mould.(Refer Slide Time: 22:32)
Let us discuss the gating design and this is one of the most important aspects of the casting;
because based on this, we can actually make lot of conclusions. Let us see how the gating
design is made.
A good gating design ensures the distribution of metal in the mould cavity at a proper rate,
without excessive temperature loss, turbulence and the entrapping gases and slags. So, these
are the points because of which we have to have an appropriate gating system and that can be
ensured by having a proper gating design.
Once again, ensure the distribution of molten metal in the mould cavity at a proper rate. Why
it is important? Because as I said in our earlier discussions that, if it is slower, then the molten
metal would not be able to reach all of the inner parts of the cavity; before it reaches the extreme
corners of the inner cavity, it will get solidified.
Therefore, the casting will have lot of defects in that case. If the gating design is appropriate,
then this thing may not happen. I said about the sprue that it has to be tapered. Once again, this
is to avoid the aspiration of the gas. There should be a well, because otherwise there will be
splashing.
So, these are the points which we will ensure by the proper gating design Second point is that
the Bernoulli’s theorem states that, the sum of the energies at the head, pressure, kinetic and
the friction energyat two points in a flowing liquid are equal.Consider this diagram. This is a simple vertical gating system and here we have the cup, then
through the sprue, the molten metal goes in to the mould. Here if you take three points - this
is the point at the level of the molten metal, up to this level, let us say, molten metal has been
poured. This is another point inside the molten metal, inside the sprue.
And here is another point 3. Let us say this distance is ht, this distance is hc and this distance
is h2 . Once again, the Bernoulli’s theorem says that, the sum of energy heads, that is the head
energy, pressure energy, kinetic energy and the frictional energy at two points, any two points
within a flowing liquid are equal. Let us consider points 1 and 3.
Here at point 1, the sum of energies will be head energy will be h1 let us say that is the distance,
height plus 1 p
ρ
; let us say 1 p is the pressure here and in this case since it is open to the
atmosphere, so it will be atmospheric pressure, . Plus
2
1
2
v
g
; so v1 is the velocity at this point ,
plus the friction energy f1 that is the friction energy at this point, this will be equal to the sum
of energies at point number 3 which is h3, which is the distance , that is the head, that is the
height .
Plus 3 p
ρ
, p3 is the pressure at this point divided by the density,
2
3
2
v
g
; v3 is that flow velocity at
this point , plus f3 is the friction here. Here you can see, h is the head; head means that height
is the distance, these distances are the called as the head.
This is in centimeter, p is the pressure, p1 and p3 pressure respectively at points 1 and 3. This
is on the liquid and given by the unit 2
N
cm
; ρ is the density of the pouring material or the
molten material .
This is given by the unit 3
g
cm
; small v is the flow velocity in cm
s
and g is the gravitational
acceleration constant, which is 981 2
cm
s , .
Now, f is the head losses due to friction and this is in cm. Subscripts 1 and 2 or 1 and 3 here
indicate any two locations in the liquid. In this case, what we are considering is the location and in the beginning, where the molten metal goes in and at this point where it enters the mould
.
(Refer Slide Time: 28:40)
In the figure the pressure at 1 and 3, p1 is equal to p3. According to Bernoulli’s theorem, the
sum of energies at any two points in a flowing liquid is the same. Level 1 is maintained
constant, here it is the pouring level. Thus, the velocity here is 0; because it is constant level.
Frictional losses let us neglect for the time being, that is, there is no friction loss neither here
nor at this point.
The energy balance equation therefore, this equation, if we consider these points that p1 is equal
to p3, v1 is equal to 0, frictional losses are neglected, then the energy balance equation between
1 and 3 if we solve that, it will be
2
3
2 t
v h
g = . If you see, ht is the distance from 1 to 3; let us say
with respect to the entrance of the mould , then the h3 will be 0.
From here we can find out the value of the 3 v , the flow velocity at the point 3, at the level 3 .
This is coming out to be 3 2 t v gh = .
Now, here of course, once again the g is the acceleration due to gravity, 3 v is the flow velocity
of the molten metal at the gate. Therefore, the time taken to fill up the mould can be obtained
by dividing this cross-sectional area Ag and the V which is the volume of the mould. If we
divide the volume of the mould by the cross-sectional area and the flow velocity which we have determined, from here we can find out the value of the tf time. The rest of the material
we will discuss in our next discussion session.
Thank you for your attention