Hello, I welcome you all in this presentation.
This presentation is related with the topic
casting and we will be talking about the different
technical aspects related with the Sand Moulding.
We know that the desired size and shape of
the casting which is free from the defect
can be achieved by preparing a mould.
Mould of the sand is prepared in case of the
sand mould casting and it should have but
to have the casting which is free from the
defect and is of the desired size, shape,
finish and desired dimensions, it is necessary
that the sand mould is having the certain
characteristics and certain properties.
So, what are the properties which are important
for making the casting with the desired size,
shape, finish and free from defects and for
this purpose, it is required that the moulds
have certain properties and certain characteristics.
Among these, like say one of the property
is, one of the characteristics is permeability.
Another is there are various conditions under
which strength is important.
So under this, we have strength of the mould
of course, there are different terms which
are used to for defining the strength depending
upon the condition of the mould like green
strength, dry strength, and each of these
strength have has their own purpose and they
play specific role and hot strength.
After this another important property is refractoriness,
then we have collapsibility.
So what is the role which is performed by
these characteristics and why are these being
considered important for making the casting
which is of the desired size, finish and free
from defects.
We know that permeability is about the kind
of pores or whites existing in the mould walls.
So through these pores, these pores actually
allow escaping of gases from the mould to
the atmosphere.
So, we know that when molten metal is poured
into the mould, before pouring there will
be air inside the mould.
Molten metal being poured will be having the
metal plus the gases in dissolved state in
the mould.
We know that at a high temperature, the gases
can dissolve into the molten metal.
So, the gases dissolved either in the furnace
or while handling these gases will be present
with the molten metal and during the solidification,
these gases are released or these are rejected.
So, these gases should come out at the same
time, the interaction between the molten metal
plus mould wall interaction also results the
evolution of the gases.
So, these gases being generated due to the
interaction between the mould and the molten
metal.
These gases also should get space for escaping
and this escaping is facilitated through these
pores which are present in the sand mould.
So, the gases will be getting the pores or
channels for coming out of the mould.
So, if the permeability or which permeability
is the characteristic which permits the escaping
of the gases present in the mould due to the
variety of regions such as the atmospheric
air or the gases in dissolved state which
are released during the solidification or
the gas is being generated due to the mould
metal reactions.
So, these gases must be in a position to escape
through the proper pores which are present
in the sand mould and if it is not so, then
all these gases will have the tendency for
entrapment.
So, the gases trapped in the mould wall within
the mould will be leading to the presence
of these gases in the casting itself and these
gases may be in presence of the fine porosity
or big blow holes.
These are also termed as gaseous inclusions.
So, these are the defects and these presence
of these defects make the casting of the lower
capability in terms of the ability to take
up the load or sensitivity for the corrosion
or the increased tendency of the failure during
the service.
So, in order to avoid such undesirable features
related with the presence of the gases in
the casting, it is necessary that the permeability
of the mould is quite good then we will come
to the another important characteristic which
is the strength.
Strength of the mould is important certainly
because whatever is the shape of the mould
that will be retained under the pressure or
metallostatic pressure of the molten metal
only if the mould is of the desired strength
and if the mould is not of the desired strength
or proper strength then under the effect of
the metallostatic pressure, the mould walls
will be damaged then this damage can occur
in the different forms.
So, since this is the molten metal filled
in the mould cavity and these are the walls
of the sand mould.
If the walls of the sand mould are not of
the requisite strength, then they will not
be able to sustain this metallostatic pressure
which will be acting on the mould walls and
under such conditions, there can be shifting
of the mould walls or there can be.
So this shift or you can say there is a change
in the position of the mould walls.
This may be all along one surface or this
may be very localized depending upon the geometry
and the weakness where does it exist.
So, if this zone is poor in terms of the strength,
then the settling of, the shifting of the
mould wall will be occurring in this region
and this will be changing the shape of the
casting, So but under what conditions strength
of the mould is checked as per the condition.
So, whenever we make the mould of the sand
using like say, sand plus moisture plus clay.
These are the three important ingredients
plus small amount of the other ingredients
are also added in order to have the desired
characteristics which are specific in nature.
So, when you see this moisture, sand and the
clay are mixed and then mould is made.
So, just after its preparation the mould whatever
strength it is there means the mould, strength
of the mould, mould strength when it has moisture,
at that state the strength of the mould is
termed as green strength.
So, green strength you see we have just prepared
a mould of the sand of the moulding sand and
it has a lot of moisture.
So in that case the strength of the mould
matters primarily for the size and shape retention
during the handling.
So, freshly prepared mould walls should not
get damaged during the handling as well as
the strength of the mould also this green
strength of the mould also matters when the
mould is used directly for pouring the molten
metal means the green sand mould is poured
with the molten metal directly without permitting
the baking or removal of the moisture.
So, in that case freshly prepared mould is
poured with the molten metal directly and
after the solidification, we get the casting.
So, in that situation the green strength becomes
important just not just from the size and
shape retention point of view, but it should
also carry the load of the molten metal.
So, the green strength of the mould becomes
important in that case.
So, green strength of the mould is the strength
of the mould when it has moisture and it is
important for the two situations, one to retain
the size and shape when the moisture is present
one and two, when and the retention of the
size and shape especially when the molten
metal is to be poured into the mould.
So, it is also required that the mould wall
should not just retain their size and shape
of by on its own weight during the handling,
but they should also take the load of the
molten metal effectively.
Another important condition in which strength
is checked is the dry strength.
So, since the green strength becomes quite
low for taking the weight of the molten metal
especially in case of the large casting.
So, it becomes important that the strength
of the mould is increased and for that purpose
normally the baking of the green sand mould
is done in the range of 150 to 350 degree
centigrade for 8 to 48 hours.
So, purpose of this is that all the moisture
is driven off and strength is increased.
Once the strength is increased, it can take
the load of the molten metal especially in
case of the large castings effectively, but
this process is long.
So, the strength of the mould under the conditions
when there is no moisture or moisture has
been driven off, at that state strength of
the mould is termed as dry strength.
Hot strength of the mould is also important
because and let us understand what is the
hot strength?
We know that whenever any mould is prepared,
whether it is green or it is dry means the
green is mould when the moisture is present
and dry mould when moisture has been driven
off.
So, means the mould is not having any moisture
but when the molten metal is poured into the
mould, the temperature of scheme of the mould
will be increasing up to the temperature of
the molten metal.
So basically the mould wall temperature rises
up to the temperature of the molten metal.
So at that, strength of the mould actually
matters for carrying the weight of the molten
metal.
So, strength of the mould walls at the temperature
of molten metal actually is important for
carrying the weight of the molten metal because
in any way whether it is green strength or
the dry strength when the molten metal is
poured, there is a rise in temperature of
the mould wall and at that state, what is
the strength of the mould that is actually
important for retention of the size and shape
of the mould.
So, hot strength is important and sometimes
special ingredients are added for increasing
the hot strength of the mould.
Refractoriness of the mould is about the ability
to sustain 
high temperature of the molten metal.
And if say this is the mould prepared of the
sand and if the skin of the mould or of the
mould sand mould does not have required refractoriness,
in that case it will start fusing as soon
as it comes in contact of the molten metal.
So, this fusion can be avoided only if the
sand mould is of the required, is having required
refractoriness means it can sustain the high
temperature without getting fused with the
molten metal because once the fusion of the
molten, mould wall starts then it will form
the part of the casting and the shape of the
casting will also get modified.
So, this is considered, this will be a situation
of the wastage of the material as well as
the change in shape of the casting.
So, it is necessary that refract the mould
is having the required refractoriness and
it depends upon the number of factors like
the type of clay has been selected, the size
of the sand grains which are there and the
temperature of the molten metal.
So, we need to see that there is a proper
compatibility in terms of the refractoriness
between the molten metal being used for the
casting as well as the refractoriness of the
mould.
Another important factor is the collapsibility.
So, collapsibility we know that of, this collapsibility
is important at this after the solidification
of the casting.
What we know that, according to this property
what we expect that the mould will collapse
or it will be crushed, collapse.
So, ability to get collapsed under either
the residual stresses or externally applied
stresses, the mould should get collapsed so
that it eases out the residual stresses which
are being developed due to the contraction
and the casting is taken out of the mould
easily after the collapse.
So, the moulding sand 
with the good collapsibility reduces the chances
of the hot tearing because during the contraction
of the casting, the mould will collapse and
it will, so that it will reduce the chances
of the hot tearing as well as it will also
reduce the tendency for the development of
the residual stresses and it will facilitate
the easy withdrawal of casting from the mould.
So this is what is facilitated in the collapsibility
is good sometimes like sawdust is added in
the moulding sand.
So, it becomes collapsible because sawdust
will be worn out at a high temperature as
soon as it comes in contact of the hot, high
temperature molten metal.
So, these are some of the properties which
are important for the sand mould in order
to prepare the castings which are free from
the defects and they are of the required size
and shape.
So, now we will be seeing that what are the
important constituents like moulding sand.
What are the constituents of the moulding
sand?
So, basically there are three important ingredients
besides some additives.
So, main ingredients 
are like silica sand or the sand then clay
and moisture.
These are the main ingredients for the moulding
sand and apart from that there are some additives.
Additives are used to increase the specific
characteristics like this may be like sawdust,
cereal, binder and the powdered graphite.
So, these are some specific characteristics
about which we will talk later on.
So basically silica sand, this is you can
say the river sand which has the silica of
92 to 96 percent normally and this has variety
of the sizes.
So, the size may range from like say few micrometer
to few millimeter and these are present in
variety of the shapes.
So, both these characteristics are important
from the moulding properties point of view.
So, shapes may be very irregular just like
this or very angular, round like this.
These are the some of the shapes and these
characteristics affect the number of properties
related with the sand mould among the clay
I will talk about the sand and the properties
which are affected by the silica sand and
then coming to the clay.
Clay provides the required strength or power
of bonding between silica grains.
It also, so this is what is their bonding
and moisture, this clay can be added is from
like say 5 to 15 percent and the moisture
is normally added 2 to 8 percent for the two
purposes.
One it activates the clay for the bonding
purpose and it offers the required so that
the activation of the clay results in the
proper bonding and when the washer is added
appropriately, it results in the desired plasticity
to the sand moulds so that desired complicated
shapes can also be realized through the addition
of these ingredients.
Coming to the different characteristics related
with the, related with these ingredients.
So, like the size of the sand particles.
So, if the size or like coarse sand particles
results in the increased permeability, increased
refractoriness, ability to sustain high temperature
is good and both these are adversely affected
with the fine size particles.
Similarly, the angular round particles, they
reduce permeability but increase the strength
of the mould but irregular shape particles
results in the increased permeability.
So, the round particles are round then permeability
is reduced because the particles get back,
get packed during the moulding effectively
which reduces the pore sand which in turn
reduces the permeability.
But the increased packing and the closer contact
between the different sand grains results
in the increased strength.
Irregular shape particles will be leading
a lot of whites and spaces between the sand
grains during the mouldings which will be
leading to the increased permeability between
the increased permeability of the mould.
Coming to the clay, clay is added as I have
mentioned 5 to 10 percent.
So, there are two types of the clay, one is
kaolinite.
This is one of the binders which has good
refractoriness of like say 1750 to 1787 degree
centigrade temperature range in which it can
sustain.
This is the melting point above which it will
be adversely affecting the mould sand properties
and another very commonly used is bentonite
clay and it is of the low melting point.
So, like melting point is like 1250 to 1300
degree centigrade.
So, for most of the low temperature metal
systems bentonite is preferred and while for
the high temperature systems or high temperature
metal of the casting, the kaolinite is preferred.
The water absorption capacity of the bentonite
is much better as than the kaolinite.
So, this results in the good bonding power
for the moulding sand.
That is why the bentonite is preferred over
the kaolinite for the low temperature casting
systems or metal casting.
Coming to the moisture that is another important
thing.
So as I have told it is 8 to, 2 to 8 percent
moisture is added and about 2 to 3 percent
actually moisture is needed for activating
the clay so that desired bonding power can
be realized for having the good strength and
for the rest of the water, remaining water
up to limited amount like say remaining 4
to 5 percent is that is 4 to 5 percent helps
in increasing the plasticity of the sand mould
of the moulding sand of the sand so that the
sand follows the curve or the geometry of
the pattern in order to achieve the fine integrate
shapes which are desired in the casting.
So, plasticity in that way is important for
realizing the shape in the mould cavity.
So here, now I will summarize this presentation.
In this presentation, basically I have talked
about the characteristics which are desired
in the sand mould and the important ingredients
of the sand, moulding sand and the way by
which these ingredients affect the moulding
sand mould characteristics.
Thank you for your attention.
