Namaskar Friends, welcome to this session
9 on our course on processing of polymers and polymer composites. Just to give a brief glimpse of what we have
covered till now, we have covered the basic properties of thermosets
and thermoplastics. We have seen the
fundamental processes of casting. We have seen thermoforming. We have seen the
process variants of extrusion, the fundamental of extrusion process and in the last session
we covered the compression molding process. So, we have covered varied types of
processes. If you see extrusion is a continuous process,
for making large long axisymmetric products, compression molding
majorly flat products. Casting for thermo
sets as per the dimensions of the mold. Thermoforming again we have seen the different
variants closed mold process. Pressure is applied through vacuum or through
pressure or through the mechanical means. So, the basic principle remains the same,
that we take a plastic, we heat it we deform it as per the shape required finally,
we let it cool. So, that it conforms to the
desired shape. Many a time the shape of the product is very
very complex. Now for the
complex product we have to device a process, we have to think that which process can
help us to make this complex product. If you can try and open a plastic covering
of a hand drill, you will see that the shape inside
the number of ribs and bosses. Those are
present there, those are the things that are very difficult to make, these days no doubt
there is there is rapid prototyping process which can help us in that. But if we want to make on large scale at a
very fast rate, in those cases I believe that injection molding process becomes indispensable. For small scale, small number of parts
only to make a few prototypes, ok, we can go for a rapid prototyping process, but in
case of large scale production we will have to
go for injection molding process. So, the basic
application area of injection molding process is where we have large scale production,
discrete parts have to be made means that is not a continuous part like a gardeners
pipe these are discrete small parts. Then the volume is very large and the shape
is very complex. In those cases, we will go for injection molding
process, which is one of the most widely used commercial plastic processing
techniques or plastic processing process. So, let us try to understand today the basic
fundamentals or the basic we can say steps involved in an injection molding process. Let us see a simulation that how the injection
molding works, and then maybe in our subsequent session, we will see what are the
different types of products that are made by injection molding process. What is the
influence of the operating parameters or the control variables on the injection molding
process? So, that we will see maybe in the subsequent
session. Today we will try to
understand the basic working principle of injection molding process. Now let us start our
discussion on the injection molding. First let us try to systematize or to outline,
what I have already explained in the introductory part of today’s session. Now, injection molding is one of the most
commonly I have told this is a commercial process industrial scale process used for
plastic components. So, it is not something
which we can say is in the research stage, already different companies are making
different types of injection molding machines. And at IIT Roorkee we have a injection
molding machine which we are using for conducting our research work. So, it is a standard commercial process that is used for
processing of plastic parts, it is used to manufacture thin walled plastic parts for
a wide variety of shapes and sizes. So, wide variety of shapes and sizes means
that it is used for very complex to fairly simple parts as well as thin walled parts
means the thickness of the parts will not be very
large which we can achieve in the compression molding process, but we cannot use very
thick parts in case of injection molding process. Maybe there are reasons for that because
we have to cool the part inside the mold cavity and that can be one of the reasons that
this process cannot be used for very thick parts. Plastic material is melted in the heating
chamber and then injected into the mold, where it cools and finally, the finished plastic
part is ejected. Now again the fundamental 3
steps are coming into picture the heating, molding or forming and then cooling. So, here you can see plastic material is melted
in the heating chamber. So, heating part is
coming injected into the mold, mold will deform the molten plastic into the desired
shape. So, the deformation is coming into picture
or the forming of the molten plastic is coming into picture. And finally, the cooling in the mold. So, that we get the final
product. So, 3 steps common are coming again into picture
that is heating, forming and cooling. Now, based upon the applications, there are
3 types of injection molding machines majorly used in industry. First one is the hand injection molding machine
operated by man a manual to use for making small plastic
parts. Then plunger type injection molding
machine as I have explained the with the help of a syringe in one of the previous
sessions. That there is a piston cylinder type of arrangement
in case of extrusion also sometime in some types of machines it is used. So, you have the molten plastic inside a barrel
and then you use a plunger or a piston to push
it out of the die. And in this case into the die in
case of injection molding machine. So, plunger type injection molding machine
and the reciprocating screw type injection molding
machine. As we have studied in extrusion
process that there will be a barrel inside there will be a rotating screw the role of
the screw you have discussed and the different
sections of the screw also we have discussed. If you remember that there is a feed zone,
then there is a melting zone and then there is a
metering zone. So, that type of arrangement exists in case
of injection molding machine also, that is reciprocating screw type injection molding
machine. The only difference that I must
highlight here between the injection molding machine and the extrusion process is or in
the in the process mechanisms is that in injection molding the plastic will be melted and
it will be pushed through either using a screw or using a piston or a plunger type of
arrangement into the mold cavity. So, here the final product will be made is
a discrete product depending upon the shape of the mold
or the die whereas, in case of extrusion same principle the plastic is in the melted
stage are in the molten stage only. And it is pushed by the plunger or it is pushed
by the rotating screw, in many cases 2 screw extruders are there. The 2 screws, but the product will come out
that die continuously. So, we will get large scale production, but
a continuous axisymmetric product can be made or we can say a continuous
product with not with much varying cross section of a constant cross section
continuous product can be made using the extrusion process. So, as learners we should keep in mind sometimes
this question is very common that what is the difference between
extrusion process and injection molding process in context of plastics, then
this should be able to answer that exclusion is a continuous process. So, we are long constant cross section products
are made, whereas, in case of injection molding, thin
walled discrete products are made may be independent products are made with thin walls. So, we will not get very thick walled products
in case of injection molding. So, basically
3 types of machines are in use. First one is a hand injection then the plunger
type injection molding and finally, the reciprocating
screw type injection molding and no need to get confused in this we will try to understand
it with the help of a diagram also. Here you have a hand injection molding machine. We can see there is a lever and we can
rotate this lever. So, this will apply this will actuate the
plunger. And then this plunger
will apply the pressure the molten plastic will be pushed into the mold cavity of the
die and it will take the shape of the die. The small plastic buttons that we use on our
shirts can be made using the hand type of injection
molding machine. So, here we can see the
red part is shown that is the final part may be that has come out of the die. So, this can be we can see most simple form
of injection molding. So, we have to heat
the plastic. We have to push the plastic into the mold
cavity, open the cavity to get the final product. So, simple type of injection molding machine. On your screen you can see a plunger type
of injection molding machine. This is a piston
or a plunger. So, our powder or the raw material is coming
from this hopper. This is the
hopper in between there are plastic pellets. Raw material. So, the raw material enters into
this cylinder and then there is heating arrangement here you can see heaters are there this
heater can be through the entire length of the cylinder. So, because of the heat the plastic will melt
and then this piston will push this molten plastic into the mold. This is the mold we can see. The mold is in 2 parts one is a
movable part another one is a fixed part. So, the fixed part is attached to the nozzle
here and the movable part can move or slide in
this longitudinal direction. Similarly, in case of compression molding
machine we have seen, here also we will have the ejector pins. This part the final part that we have made
this blue colour part you on your screen we can see this part is fabricated. This part will move along with the
movable part of the mold. And finally, the ejector pins will give a
slight tap on the part. So, that it is pushed out or it is released
from the mold that is a movable part of the mold. So, we will see what are the various steps
involved, but here in this case the major thing
that needs to be emphasized is the injection mechanism. Here we are using a piston type
of injection mechanism. So, piston will force this molten plastic
through this nozzle into the mold cavity. So, the mold cavities created by 2 mold halves. One is a fixed mold half
and another one is a movable mold half. So, this is one thing. In second case there is a rotating screw,
that pushes the molten plastic through the nozzle
into the mold cavity. This is the mold 2-part mold here, and then
this slides over these 2 tie bars. The movable part of the mold will slide over
these 2 tie bars and then maybe it will move in longitudinal direction. So, when it will come close to the fixed part
of the mold, the mold will get closed in between we will have our mold cavity. And that mold cavity will be the exact replica
of our final product that we want to make. So, in this case our we can say injection
mechanism is a screw type of injection mechanism. So, this is a screw inside our barrel. So, here is a screw and the barrel has a heating
arrangement. Here we have the heaters
which will heat or melt the plastic. The raw material is again coming from the
hopper. So, the only difference between a screw type
of injection as well as the plunger type of injection is in the mechanism through which
we are pushing the molten plastic into the mold cavity. In piston type, we will use a piston or a
plunger inside the cylinder to force the plug molten plastic into the mold cavity,
where as in case of screw type we will use a
rotating screw to force the plastic or the molten plastic into the mold cavity we will
try to understand this. With the help of a animation also. So, that is the basic difference. So, this is on your screen reciprocating screw
type injection molding setup. Now let us
try to understand the basic working principle of injection molding process. Plastic
materials usually in the form of powder or pellets are fed from hopper into the injection
chamber. There is a hopper in which we will put our
pellets and this will then come in to the injection chamber which can be a barrel. So, a piston and cylinder reciprocating
screw. So, piston and cylinder is one type of mechanism
reciprocating screws and other type of mechanism which we have understood
in our previous slides. So, any of the
mechanism that type of arrangement is used to forward or push the material inserted
from the hopper into the injection chamber. So, that these 2 arrangements are there as
we have already seen. The material is heated in
the injection chamber with the application of heating element. So, that also we have
understood. The molten plastic material is then injected
into the mold through the nozzle. That also we have understood that first the
raw material in the form of pellets or powder is will come from the hopper it will enter
in to the injection chamber the injection chamber as a heating arrangement all through
the periphery. Now because of the heat coming from the heaters,
the pallets will melt and these molten plastic will then be pushed either using a
piston cylinder arrangement or using a reciprocating screw type arrangement. And this will be pushed through the nozzle. So,
we have reach the nozzle that is one part where we have the molten plastic now. So, what are the various steps let us go back we have
an injection chamber in which we have 2 arrangements either a piston or a cylinder
or a reciprocating screw. Prior to that we have
a hopper from whichever raw material in the form of pellets is moving down. Now this
pellet or solid raw material because of the heating arrangement around the barrel will
melt and this molten plastic is fed through the 2 different types of arrangement
depending upon the types of machine that we are using at nozzle. Now what is going to
happen at nozzle we are pushing there is a pressure from backside. So, because of this pressure the plastic will
flow and enter into the mold cavity. Or we
should use the word injected into the mold cavity because the name of the process is
injection molding. So, the material now through the nozzle will
be pushed into the mold cavity. So, the molten plastic is then injected into
the mold through the nozzle, that the molded part is cooled quickly in the mold,
there are cooling arrangement or cooling coils or cooling ducts around the mold which will
form a forced cooling type of system. Why?
because in the whole process we need to cool the final product. Why in many cases, in
most of the cases injection molding will be used for thermoplastic type of material and
as we have seen in our previous session. On compression molding also, that for complete
curing or complete molding or completing the process we need to cool the
from the thermoplastic parts. In case of
compression molding also we have the cool the plastic part, therefore, it is not widely
used for thermoplastics because the process cycle is very long. Similarly, as I have
already highlighted injection molding process is used for thermoplastic parts only and
thermo plastic parts needs to be cooled before they are finally, ejected out of the mold.
and therefore, we need to accelerate the rate of cooling and therefore, we will have a
cooling system inbuilt in our machine around the mold, we may have may be cooling
jacket or cooling coils or maybe cooling chamber in which whether air circulation or
water circulation is provided depending upon the type of thermoplastic that we are using. So, the molded part is cooled quickly in the
mold. So, quickly means we have an
artificial arrangement of cooling. The final plastic part is removed from the
mold once the process is complete. The final plastic part will be removed. And we will see in the animation that how
the part is removed from the mold cavity. The process cycle for injection molding is
very short means it is very very fast process. Typically, between 2 to 60 second. So, you can see
may be we can make number of products in 1 hour, may be 60 seconds means 1 minute. So, in 1 hour we can make 60 different parts
using the injection molding process. Now, basically the whole injection molding
cycle can be divided into 4 important steps. . So, the complete injection molding process
is divided into 4 stages or 4 steps. We can see
we will try to understand all these 4 steps one by one. So, first one is clamping then
injection then cooling and then ejection. Now clamping means the closer of the 2 parts
of the mold. Now in this case we have one fixed part of
the mold that is connected to the nozzle. There is a movable part that moves over the
tie bar. As soon as the process starts
the mold is in open stage. Now we will discuss just one cycle of processing
or fabricating one part. So, one mold half is attached to the nozzle. Other part will open up and the product will
be removed. Now the next cycle will start. This movable part will move it will come
close to the mold, fixed part of the mold. So, when these 2 halves of the mold will close
down we will have the cavity cut inside which is the exact replica of the final product
that we want to make. So, the fixed part of the mold, the movable
part of the mold close down together. So, that step or that stage is called as the
clamping. Now we have the
mold cavity inside the 2 halves of the mold and the mold as closed it has been clamped. So, clamping force is required to keep the
mold clamped. So, first stage is clamping. Once we have the mold cavity inside the 2
halves of the mold. The next stage is injection. So, already through the hopper the pellets,
polymer pallets have travelled down into the barrel or the
injection chamber. Because of the heaters
provided there, the molten plastic has been heated. It is maintained in the molten stage. and because of the screw or the piston cylinder
type of arrangement it is pushed through the nozzle into the mold cavity. And that stage or that step is called as the
injection. So,
first we have close the mold, then we have injected the molten plastic into the mold. Now we will keep the mold closed there will
be air or water circulation around the mold. So, that the cooling is accelerated. So, the third stage is cooling, it can be
natural cooling dependent upon the requirement it can be forced
cooling through the water circulation or the air circulation. Finally, once the product has solidified inside
the 2 mold halves, the movable half of the mold will move back and it is desirable that
the plastic part made inside the 2 halves of
the mold travels with the movable the half of the mold. And finally, there are ejector pins
as was the case in case of compression molding. These ejector pins will give a slight tap
on the plastic part and the plastic part will be detached from the movable half of the mold
and it can fall down into the collecting bin or the bucket or the box. So, these are the 4 stages one is the clamping closing of the
mold injection of the molten plastic cooling of
the plastic part. And finally, the ejection of the plastic part
form the mold cavity. So,
these are the 4 stages. I have tried to explain to the best of my
ability. And now will try to read these 4 stages
what has been put in the form of slides. Now, clamping. 2 halves of the mold must be tightly closed
before the molten material is injected into the mold. One half of the mold is attached to the injection
unit that is nozzle and the other half is allowed to slide on
the guide ways or the tie bar. The clamping of the mold is operated hydraulically,
which pushes the moving half of the mold towards the fixed. Half of the mold to make a airtight chamber. So, 2 half of the
mold will close and then we inside we will have your mold cavity. The pressure and a
time required to close and open the mold depends upon the machine capability. So, each machine has specifications at what
is a maximum clamping force that machine can apply. Now once the mold has been closed, then we
inject the molten plastic the plastic material is melted by the application
of heat and forwarded through the piston towards the nozzle and finally, into the mold
cavity. The molten plastic is then injected into the
mold cavity. The amount of material that is
injected into the mold is referred to as short volume. Now this is a new term that is
coming in our discussion that is short volume. Now, short volume is the metered amount of
molten plastic that we want to inject inside the mold cavity. So, the amount of material that is injected
into the mold through the nozzle is called as the shot volume. The injection time can be estimated by the
shot volume, injection power and pressure. So, we can control the pressure that we are
applying to force the molten plastic into the
mold cavity and we will see the influence of each of these parameters on the quality
of the injection molded part. In our next session today we are trying to
just understand the basic stages of the injection molding process. So, our injection tangent for how much
time we will inject the molten plastic that will depend definitely it will depend on shot
volume as well as the pressure that we are applying to the molten plastic to force it
into the mold cavity. Now let us try to understand the injection
molding through the process of simulation. So, here we have very good simulation for
the injection molding process. www dot
tronicarts dot de. So, this is the pellets coming down this is
the hopper. These are the heating elements heating elements,
4 heating elements around the injection chamber. So, this is a showing a reciprocating screw
type of injection molding machine, metering zone, transition zone. So, these are the pellets red colour pallets
that are coming down. Now, the screws rotating and the material
is moving down the cause of these heating elements it will get melted and this is a
nozzle. So, here with this is the mold. The mold is closed. We can see this is a closed mold. And these are the blue colour
portion are the cooling coils now this has forced the red colour plastic into the mold. this is a fixed part of the mold, this is
the movable part of the mold. This is the final
product that we want to make. So, this is movable part of the mold has moved
and the product has been released. We will see another injection process. Again it has been filled. The movable part will move back movable part
of the mold. It
has moved back and it has been pushed out. So, you can see I think the process, now is
very very clear to all of you, that from the hopper the raw material is coming into the
injection chamber. Then there is a then there
is a rotating screw type of arrangement. It is forcing the plastic or the molten plastic
through the nozzle into the mold cavity. The heaters are attached all around the barrel
or the injection chamber, which heats the molten plastic, melts the molten plastic. And once it is injected into the mold cavity
the cooling circulation of water or air cools
the part inside the mold cavity only. And once
the part has solidified after a particular holding time the movable part of the mold
will move back the product will also move along
with the movable part, and then the ejector pins will just give a little tap on this part
that has that has been formed. And it can be
then finally, collected. So, this was the continuous process and you
can get large number of parts maybe 50 to 60 parts in an hour if
the sizes, shot volume is very less or may be
small volume parts can be even larger number can be produced on an hourly basis. Now, let us see what are the different types
of materials used for injection molding process. Majorly used for thermoplastics we can see
acetyl acrylic, nylon polycarbonate, polyetheretherketone, polyethylene, polypropylene,
polyvinyl chloride. So, majorly used for thermoplastic parts. What are the applications? Now, the injection molding process can be
used to manufacture thin walled plastic housing products, which require many ribs
and bosses on the interior surfaces. Now if
you remember in compression molding, we have discussed the ribs and bosses or 2 many
ribs and bosses are not possible in case of compression molded parts, but those are
possible in injection molding parts. These housings are used in a variety of products
including household appliances, electronics, power tools and as automotive
dashboards. So, you can see that these
housings that we can make using injection molding process, find a number of
applications. Thin walled products include different types
of open containers or such buckets. So, we use plastic buckets in our household. So, there are made generally by the
injection molding process only. It is also used to go to several daily use
items such as toothbrushes or small plastic toys many medical
devices including valves and surgical syringes. So, injection molding process has got wide
range of applications and different types of parts even the rear view mirror of
our motorbikes can be made by using the injection molding process. Now, what are the advantages of injection
molding process. It has got higher production
as we can see the number of parts produced per hour are very large. Even a single minute
we can produced one part. Close tolerance is on small intricate parts. So, difficult to
make the parts that we had which are difficult to make using the other processing
techniques or plastic can be made using the injection molding parts. So, part complexity
is not a limitation only thing, that if the part is very very complex it is very difficult
to fabricate the die or the mold for that part,
but once the die and the mold is possible or it
can be fabricated it is it then becomes a very fast process of processing of complex
parts also. So, the may wastage of material is less, complex
geometric and easily as I have already highlighted. This process is suitable for difficult to
process complex geometries. What
are the limitations now? As I have already told that the mold is the
most important part of injection molding machine and the fabrication
of molds is one of the limitation. So, the tooling cost is very very very high. For example, if you have seen the plastic
chairs that we use in our day to day life, the mold cost of that chair is very very large. So,
the mold is the most important part. Once we are able to fabricate the mold then
the process is fairly simple. Set up cost is also high as well as large
undercuts cannot be formed. So, each process has got it is own limitations
that the kinds of ribs and bosses which cannot be formed in other plastic molding
techniques can easily be formed using the injection molding technique. So, with this we come to the end of this session
on injection molding. In next session also we will discuss the finer
details even a little bit of we can say the influence of the various parameters
of injection molding on the process, that how
we can control the parameters of injection molding process in order to make a good
quality part using this process. So, in injection molding second session we
will discuss the finer details of the process as well as
the discussion on the operating variables. Thank you.
