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Various Aspects of Casting

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Manufacturing Processes – Casting and Joining
Prof. Sounak Kumar Choudhury
Department of Mechanical Engineering
Indian Institute of Technology Kanpur
Lecture – 02
Hello and welcome back to the course on Manufacturing Processes - Casting and Joining. Let
me remind you - we were discussing the casting process and we started discussing about the
advantages and disadvantages.
We said that, the greatest advantage that we get from the casting is that there is no wastage of
material like in case of machining where we waste a lot of material in terms of the small chips,
which cannot be reused.
(Refer Slide Time: 00:53)
In case of casting, some casting processes are capable of producing parts to net shape. Other
casting processes are near net shape type for which some additional shape processing will be
required. Usually, they are machining in order to achieve accurate dimensions and details, this
we said last time.
Casting can be used to produce very large parts. Casting weighing more than 100 tons have
been made. More than 100 tons means, as I gave you an example of the Buddha statue in the
Spring Temple in China - this is 1000 tons of weight and there we have about 1100 different
parts, each of them is made by casting process.So, as you understand that each of them is about few tons. Casting processes can be performed
on any metal that can be heated to the liquid state. So, this I said earlier, that any metal that can
be melted can be casted also.
Some casting methods are quite suited to mass production. This means that those casting
processes are easy to automate and they can be actually used in the mass production at a very
large scale; because of the automation the production rate goes very high.
(Refer Slide Time: 02:35)
Now, there are few disadvantages as well. The biggest disadvantage, I already told you
indirectly, is the limitation on mechanical properties. Mechanical properties are not always
very good.
Porosity: when I was introducing the casting process, I told you that the gas which evolves
from the cavity after the molten metal is poured into the cavity has to be escaped.
The mold material has to be selected in such a way that the gas can be actually taken out, it can
escape. If it goes through the molten metal and cannot escape, it remains inside the molten
metal in terms of small air bubbles and those create the porous surface in the casting. Poor
dimensional accuracy and surface finish for some casting processes for which we need to have
the subsequent machining or other processes. Safety hazards to humans when processing hot
molten metals and environmental problems.Because, always there will be some fume coming out. Apart from that, it will be tremendously
hot, because as you understand that the process deals with the molten metal. So, temperature
becomes very high in the foundry shop where the metal is melted, or the casting process is
being performed.
(Refer Slide Time: 04:22)
Now, there is a historical background, which is interesting to know The casting is one of the
oldest shaping processes, dating back 6000 years. That means, 6000 years ago, people knew
about casting. Discovery of casting copper occurred accidentally in Mesopotamia.
Initially various kinds of ornaments used to be made by copper. And, the ornament which used
to be made by copper, used to be made by hammering. So, the proper shape was given by
hammering only.
Therefore, as you understand the quality and the accuracy was not very good. Now, the casting
of copper started being introduced, shapes much more intricate could be formed by casting than
by hammering, which was used earlier to get the shape. More sophisticated tools and weapons
could be fabricated. Because casting processes were introduced, more sophisticated tools were
introduced.
More detailed implements and ornaments could be fashioned. Fine gold jewellery could be
made more beautiful and valuable than by previous methods, that is the hammering. So, that
was the advantage almost 6000 years back.Egypt ruled the western civilized world during the Bronze Age, which is about 2000 years,
largely due to it is ability to perform the casting processes. They already knew, they are aware
of different kind of casting processes and therefore, they could fabricate very intricate shape of
tools, very fine jewelry and that is how they actually ruled the western civilized world for a
very long time.
Construction of cathedrals, churches required the casting of bells, the huge bells which are
there on the cupola of the churches or the cathedrals.
These bells actually are made by casting. Particularly the sand casting because there is no other
way you can make such huge and such heavy bells. Now, many of the casting techniques
developed for bell founding were applied to cannon making.
So, when the bell founding or the foundry of the bell- that means, making the bell by casting,
was developed then they started using it for making the cannons for the defense. Now, it was
very popularly used because the cannons could be made at a faster rate and at a larger number,
that was important at that time.
The first cannon, which was cast, made of bronze, that was made in the Ghent, which is in
Belgium. And, surprisingly this is in the year 1313 just imagine how long ago. And, another
surprising thing is that this was made by a religious monk. The first cannon, which was a
casting made by a religious monk was made by using the same technique that was used for
making the bells.
The list of parts includes dental crowns made by casting, jewellery, statues, wood-burning
stoves, engine blocks, heads for automotive vehicles, machine frames, railway wheels, frying
pans, pipes, pump housings. So, these are the different types of parts which are made by casting.
All varieties of metals can be cast, ferrous and nonferrous. These are the greatest advantages
of the casting process.(Refer Slide Time: 09:11)
It is interesting to look at the historical background. This is an example of the casting that all
of you must be knowing. It is the Ashok Pillar and this Pillar is located in Delhi o, this is in
268-232 before Christ, made by the Mauryan Emperor Ashoka. So, just imagine how long ago.
Now, it is 2020 and then add another 232 . And, this is 7.2 meters high, 16 inch diameter and
weighs 3000 kilogram. Now, just imagine that such a huge pillar, how this could be actually
made by casting.
Another surprising thing that all of you must be knowing that the material, which material, it
is made of that is not very clear so far. Because, it is still not rusted although it is lying under
the open sky, and it has gone through the rain and the air but it is not rusted yet. So, just imagine
that it is about 2252 years ago that this pillar was actually made.(Refer Slide Time: 10:34)
Application of aluminum die casting in the automobiles is shown here and you can see the
different parts are labeled here in this diagram. You can see that many of these parts are made
by the casting.
Those parts are made of aluminum material, because in automobiles we try to get the weight
as less as possible, for the less fuel consumption. These aluminum parts are made by the die
casting and the die casting method will be discussed in great details later on.
(Refer Slide Time: 11:13)Foundry practice: foundry practice is the casting process that is called the foundry. Following
areas need attention for the successful casting operation so that you could get a casting. By the
way, the final product, which you are getting out of casting process is also called the casting.
So, I will be using frequently the casting as a process as well as the casting as the final product.
So, to get the final product to get the casting, which is of good quality, which is defect free,
you need to have these four points very scrupulously kept in mind. First of all, this is
preparation of molds and patterns. Second point is the melting and pouring of the liquefied
metal. Solidification and further cooling to room temperature and the casting defects and the
inspection.
I told you about these points a little bit, that what kind of molds you will be using, what kind
of sand you will be using, how much is the strength of the mold, how much is the porosity of
the mold, what kind of patterns you will be using, whether it is the single use, once use or it is
the multiple use patterns and so on. Melting and pouring of the liquefied metal, what is the
pouring temperature.
Because depending on that you will have different kind of castings, that is you may have a
casting with a defect, where the molten metal does not reach the intricate parts of it because
the initial pouring temperature was less, was not adequate. During solidification and further
cooling to room temperature, different kind of shrinkages occur.
When it solidifies, it actually reduces in size, either it is in the dimensional or it can be in the
taper form, for example, that we will see at a later stage. Casting defects and inspection should
be looked into very carefully. So, all those things you can see. I will show you right now a
video on the foundry practice and you can see these processes clearly.
So, let us go through a small video clip, where you will see all those things in details and I
think it will be very clear to you how the entire process, entire foundry practice is performed.(Refer Slide Time: 13:51)
Castings a vital necessity in all engineering industries, some simple others more complex, many
of them are hollow like this typical pipe T piece.
(Refer Slide Time: 14:03)(Refer Slide Time: 14:12)
(Refer Slide Time: 14:15)
How are such castings made? From the original machine drawing of the required part a wooden
pattern is developed.(Refer Slide Time: 14:20)
Frequently it is designed in two pieces to assist the molding processes.
(Refer Slide Time: 14:27)
.(Refer Slide Time: 14:31)
Dowels in one part and sockets in the other, ensure perfect registration when the two pieces are
put together.
(Refer Slide Time: 14:40)
Once, it has been designed and made a pattern may be used many times. (Refer Slide Time: 14:47)
(Refer Slide Time: 14:52)
Here, we see a hand molder putting down the bottom half of the pattern onto the wooden board
that will form a temporary support.(Refer Slide Time: 14:58)
(Refer Slide Time: 15:00)
When the mold box is filled with special molding sand, that is carefully rammed home around
the pattern.(Refer Slide Time: 15:09)
The success of hand molding is entirely dependent upon the skill of the molder whose
knowledge and judgment determine how tightly the sand should be rammed up.
(Refer Slide Time: 15:12)
.(Refer Slide Time: 15:30)
After removal of surplus sand and squaring off the face of the lower part of the mold, the second
part of the pattern is fitted on.
(Refer Slide Time: 15:36)(Refer Slide Time: 15:42)
Dry parting sand on the mold face ensures clean separation of the two halves of the mold later.
(Refer Slide Time: 15:47)(Refer Slide Time: 15:56)
The two mold boxes are kept in correct alignment by register pins. After separation, the patterns
are withdrawn from the two half molds, the bottom half of the mold is called the drag, the top
half the cope.
(Refer Slide Time: 16:07)(Refer Slide Time: 16:13)
The pattern is first wrapped to clear it and give an easy withdrawal.
(Refer Slide Time: 16:27)
For a hollow casting a core of baked sand is used to prevent the molten metal from filling the
whole space left by the pattern in the mold.(Refer Slide Time: 16:39)
(Refer Slide Time: 16:51)
Here you can clearly see where the metal will flow to form the walls of the casting. The register
pins make it possible to replace the top part of the mold in its correct alignment.(Refer Slide Time: 16:59)
(Refer Slide Time: 17:06)(Refer Slide Time: 17:27)
The molder also controls the pouring of the metal. Later he breaks down the mold, the core
sand is broken out from inside the hollow casting, which is then fettled and machined.
(Refer Slide Time: 17:41)
And, we have the finished job.
So, you have seen the foundry practice video. I hope this process is very clear to you. What
was shown is the pattern, how the pattern can be inserted, how the cavity can be made and how
the internal cavity can be made by the use of different kind of cores. And, this was the sand casting. So, a cavity was obtained within the sand mold. And, hence when the final product or
the final casting was obtained, that mold had to be broken.
Therefore, it cannot be used more than once. All these 4 points, namely preparation of molds
and patterns, melting and pouring, solidification, casting defects are important. Casting defects
of course, have not been shown, but I will discuss casting defects at a later stage.
(Refer Slide Time: 18:48)
Now, I would like to show you another very interesting video, which is the video on the making
of the iron wheel. This is little similar to the earlier one. Nevertheless, it will actually give you
a very good idea about the casting process.
This is a very small clip and here is that, which is the making of the iron wheel and these iron
wheels are made for the BMW Cars. So, you can see the high quality that they obtain out of
the casting process. Here is this.(Refer Slide Time: 19:27)
(Refer Slide Time: 19:32)
In another video, I have made a casting model or pattern on my lathe. Now, it is time for
showing the casting process. This is done by the company near Unsauro the foundry from
Copenia. It is a really beautiful and industrious place and I am glad that I got the permission to
film here.(Refer Slide Time: 19:42)
(Refer Slide Time: 19:47)(Refer Slide Time: 19:52)
(Refer Slide Time: 19:56)
First part of the process is of the preparation of the mold. The bottom half of the molding box
or flask is set onto the floor, where one part of the pattern is laying.(Refer Slide Time: 20:02)
.
The oddly shaped block in the corner will form a cavity to hold a special filter to keep the slag
out. The other blocks are sprue patterns, they form the channels, which will lead the molten
iron to the mold cavity.
(Refer Slide Time: 20:24)(Refer Slide Time: 20:29)
And, here comes the big sand dispenser. The sand is poured into the flask and pushed by hand
into the corners.
(Refer Slide Time: 20:49)
The flask is filled to the rim and after some compacting the top is trimmed flush with the rim.
After 10 to 15 minutes the resin that is mixed with the sand has hardened. So, the mold can be
turned upside down.(Refer Slide Time: 21:15)
(Refer Slide Time: 21:34)
It is now in the same position as it will be later for pouring. After cleaning away the dust the
top molding box the cope is set upon of the bottom one the drag.(Refer Slide Time: 21:59)
Then, the other half of the pattern is put in place, also the other parts of the sprue pattern and
an internal feeder.
(Refer Slide Time: 22:16)
The graphite powder is used to make it easier to separate the patterns from the mold later; on
the left an iron pipe is visible, that is the pattern for the vertical parts of the sprue.(Refer Slide Time: 22:48)
The internal feeder is covered with sand completely. It is just the cavity with an opening
towards the pattern.
(Refer Slide Time: 22:58)
The iron will fill this space and provide additional liquid iron when the molten iron in the wheel
is cooling down and shrinking.(Refer Slide Time: 23:02)
Now, the halves of the mold are parted to remove a pattern.
(Refer Slide Time: 23:16)
The two protruding knobs that are visible on the top part are there to interlock the two parts in
the right position.(Refer Slide Time: 23:30)
So, that is the moment when my threaded inserts in the model become important. Three hooks
are screwed into them and with carefully knocking upwards, the pattern will eventually come
loose. Of course, the pattern has to have no undercuts and the vertical surfaces need to be
slightly tapered.
(Refer Slide Time: 24:02)(Refer Slide Time: 24:07)
(Refer Slide Time: 24:36)(Refer Slide Time: 24:39)
When we finally put the model of the wheel aside after 6 times of molding; it looked quite ok
at first.
(Refer Slide Time: 24:44)(Refer Slide Time: 24:49)
But a closer look revealed some damage; if I had wanted more wheels I should have used better
wood.
(Refer Slide Time: 24:52)
Next step is the preparation of the mold for the casting. This reddish liquid is a suspension of
shamud, it will form a thin layer on the mold as a protection against the heat of the liquid metal.(Refer Slide Time: 25:03)
(Refer Slide Time: 25:17)(Refer Slide Time: 25:26)
Because it is alcohol based it can be set on fire. So, it dries and hardens at the same time.
(Refer Slide Time: 25:35)
Now, the filter is inserted, and some fireproof putty is laid around the cavities in the mold.(Refer Slide Time: 26:03)
Now the top is set upon the bottom and the two parts are pressed together with clamps.
(Refer Slide Time: 26:24)
.(Refer Slide Time: 26:31)
The second part is of the casting process itself. Here is the liquid iron in an induction furnace,
slag is removed from time to time.
(Refer Slide Time: 26:42)(Refer Slide Time: 26:46)
(Refer Slide Time: 26:50)
But the furnace is not full yet. So, another container of scrap metal goes into it; maximum load
is 3.5 tons.(Refer Slide Time: 27:12)
When the temperature is above 1350 degrees Celsius, the iron is filled into a casting ladle and
brought to the molds.
(Refer Slide Time: 27:37)
.(Refer Slide Time: 27:44)
The final measurement of the temperature, when about 80 kilograms of cast iron are poured
into the mold.
(Refer Slide Time: 28:13)(Refer Slide Time: 28:18)
(Refer Slide Time: 28:38)
While cooling down, the iron and the other components, mostly carbon, will form microscopic
crystals. And, their shape influences the material properties; this will be ordinary cast iron with
lamellar graphite also known as grey iron.(Refer Slide Time: 28:44)
.
(Refer Slide Time: 28:51)(Refer Slide Time: 28:54)
Some days later, I came back and looked for the wheels in this part of the factory where the
paddling and finishing is done.
(Refer Slide Time: 29:01)(Refer Slide Time: 29:05)
Oh, here they are, 45 kilogram each and already sandblasted. Next, we will have to drill the
holes into them, then they will be ready for use in our cannon project.
So, now, I think you have a clear idea about the casting process, particularly the sand casting.
In these two videos we have watched different aspects. You have to see this carefully, because
lot of things which I already told you earlier have been shown to you in these video clips.
Particularly in the second part of second video, you must have observed that there are some
waste materials, which are the materials from the gating system which is cut off from the final
casting; they are actually remelted.
You have seen that those parts are again put back to the oven and they are remelted. So, I am
coming back to my earlier advantage point of casting that here the wastage of material almost
is none. So, that is one of the greatest advantages.
And, of course, you have seen the use of the core. So, the core is used to get the internal
intricacies, internal cavities, that also have been shown in the first video; you must have seen
that. Overall, this gives you a complete picture of the sand mold casting. And, about the other
castings, I will show you few more videos, small clips. About the other casting processes, we
will be discussing in our subsequent discussions.
Thank you for your attention