This document provides information about the extrusion process. It begins with an introduction and then describes how extrusion works, including how plastic material is fed into a heated barrel and screw to become molten plastic, which is then forced through a die to shape the final product. It discusses advantages of extrusion and provides details about single screw extrusion mechanisms and flow analysis. It also classifies different types of extrusion processes and discusses factors that affect extrusion force.

1. Aadrsh Kumar Tiwari
(M.Sc. Polymer chemistry)
Roll no.-131021
Enrollment no.-1750016
Extrusion

2. Content
Introduction
Extrusion
Advantages
General features of single screw extrusion
Mechanism flow
Analysis of flow in extruder
Classification of extrusion processes
Die materials
Factor affecting the extrusion force

3. Extrusion molding is most common method
employed to process thermoplastic materials
into continuous sheeting, film, tubes, rods and
filaments.
It is also used to coat wire, cable, cord and
also mixing device.
Other applications are reprocessing of waste
materials for making the granules.
In general, extrusion is used to produce
cylindrical bars or hollow tubes or for the
starting stock for drawn rod, cold extrusion or
forged products.

4. Extrusion
In extrusion, dry plastic material is first loaded
into a hopper, and then fed into a long heating
chamber (barrel) through which it is moved by
continuously revolving screw.
Molten plastic is forced out through a small
opening called die, which gives the desired
shape to the product.
As the extrudate comes from the die, it is
cooled, by air-blowers or by immersion in water.

5. Melted thermoplastic can be woven into fabrics for
making cloths, curtains, and carpets.
In wire & cable coating, thermoplastic is extruded
around a continuing length of wire or cable, which like
the plastic, passes through extruder die.
The coated wire is wound on drums after cooling.
In the production of wide film or sheeting, plastic is
extruded as a tube.
This tube is made to split as it comes from the die and
then stretched and thinned, as per the desired
dimensions.

6. Advantages
There are number of advantages of using extrusion process which
are following:
By increasing forming pressure, we can get thinner walls
The cost of extrusion dies is less as compare to the dies of other
processes
The operation cost of this process is low
Large reduction of area leads us for easier secondary operation
Extrusion process gives us more flexibility in the design of metal
We get good dimensional tolerances from this process
Use of spider die will allow us to create internal impression in the
metal
Extrusion process is excellent to give us brittle products

7. General Features of Single Screw Extrusion
One of the most common methods of processing
plastics is Extrusion using a screw inside a barrel.
The plastic, usually in the form of granules or powder,
is fed from a hopper on to the screw.
 It is then conveyed along the barrel where it is heated
by conduction from the barrel heaters and shear due to
its movement along the screw flights.
The depth of the screw channel is reduced along the
length of the screw so as to compact the material.
At the end of the extruder the melt passes through a
die to produce an extrudate of the desired shape.
 As will be seen later, the use of different dies means
that the extruder screw/barrel can be used as the basic
unit of several processing techniques.

8. Basically an extruder screw has three different zones.
(a) Feed Zone The function of this zone is to preheat the plastic and
convey it to the subsequent zones.
(b) Compression Zone In this zone the screw depth gradually decreases so
as to compact the plastic.
This compaction has the dual role of squeezing any trapped air pockets
back into the feed zone and improving the heat transfer through the reduced
thickness of material.

9. (c) Metering Zone In this section the screw depth is again
constant but much less than the feed zone.
 This zone is the most straight-forward to analyse since it
involves a viscous melt flowing along a uniform channel.

10. Mechanism of Flow
As the plastic moves along the screw , it melt by the
following mechanism.
Initially a thin film of molten material is formed at the
barrel wall.
As the screw rotates, it scrapes this film off and the
molten plastic moves down the front face of the screw
flight.
When it reaches the core of the screw it sweeps up
again, setting up a rotary movement in front of the
leading edge of the screw flight.
Initially the screw flight contains solid granules but
these tend to be swept into the molten pool by the rotary
movement.

11. As the screw rotates, the materials passes further
along the barrel and more and more solid material is
swept into the molten pool until eventually only melted
material exists between the screw flights.
In one case the material sticks to the screw only and
therefore the screw and material rotate as a solid
cylinder inside the barrel.
In the second case the material slips on the screw and
has a high resistance to rotation inside the barrel.
This results in a purely axial movement of the melt and
is the ideal situation.
The external heating and cooling on the extruder also
plays an important part in the melting process.

12. As discussed in the previous section, it is convenient to consider
the output from the extruder as consisting of three components-
drag flow, pressure flow and leakage.
The derivation of the equation for output assumes that in the
metering zone the melt has a constant viscosity and its flow is
isothermal in a wide shallow channel.
These conditions are most likely to be approached in the
metering zone.
a) Drag Flow: consider the flow of the melt between parallel
plates as shown in fig…(a)
For the small element of fluid ABCD the volume flow rate dQ is
given by:
dQ= V * d y * d x………. (1)
Assuming the velocity gradient is linear, then

13. Substituting in (1) and integrating over the channel depth, H,
then the total drag flow, Qd, is given by:
…………(3)
This may be compared to the situation in the extruder where the
fluid is being dragged along by the relative movement of the screw
and barrel.
The position of the element of fluid and (2) may be modified
to include terms relevant to the extruder dimensions.
For example
Vd = nDNCOS4

14. In both cases, A6 = dz, element width = dx and channel width = T
Fig. 4.7 Melt Flow between parallel plates

15. Pressure Flow: consider the element of fluid shown in fig.(b).
where N is the screw speed (in revolutions per unit time).
In most cases the term, e, is small in comparison with so
this expression is reduced to
Note that the shear rate in the metering zone will be given by
Vd/H.
……………..(3)
where P is pressure and d t is the shear stress acting on the
element. For steady flow these forces are in equilibrium so they
may be equated as follows:

16. F1 = F2 - 2F3
which reduces to
Now for a Newtonian fluid, the shear stress, ty, is related to the
viscosity, q, and the shear rate, 9, by the equation
…………..(4)
Using this in equation (4)
Integrating…

17. so
Also, for the element of fluid of depth, dy, at distance, y, from the
centre line (and whose velocity is V) the elemental flow rate, d e ,
is given by
dQ = VTdy
This may be integrated to give the pressure flow, Qp
…………..(5)
…………….(6)
This equation may be rearranged using the following substitutions.
Assuming e is small, T = D tan ø. Cos ø
also

18. Thus the expression for Qp becomes
……………….(7)
(c)Leakage: the leakage flow may be considered as flow
through a wide slit which has a depth, a length (e cos ø) and a
width of Since this is a pressure flow, the derivation is
similar to that described in (b). For convenience therefore the
following substitutions may be made in (6).
So the leakage flow, QL,
is given by
……………….(8)

19. A factor is often required in this equation to allow for eccentricity of the screw
in the barrel. Typically this increases the leakage flow by about 20%.
The total output is the combination of drag flow, back pressure
flow and leakage. So from (3), (7) and (8)

20. …….(9)
For many practical purposes sufficient accuracy is obtained by
neglecting the leakage flow term. In addition the pressure gradient
is often considered as linear so
where 'L' is the length of the extruder. In practice the length of
an extruder screw can vary between 17 and 30 times the diameter
of the barrel.
The shorter the screw the cooler the melt and the faster the
moulding cycle.
In the above analysis, it is the melt flow which is being
considered and so the relevant pressure gradient will be that in
the metering zone.

21. CLASSIFICATION OF EXTRUSION
PROCESSES
1.By Direction
Direct /Forward
Indirect /Backward
2.By Operating
Temperature
Hot Cold
3.By Equipment
Horizontal
Vertical

22. Direct Extrusion
It is a process in which the metal billet is placed in a container
is forced by a ram to pass through a die.
In this type the direction of flow of polymer is in same as that of
movement of ram.
The punch closely fits the die cavity to prevent backward flow
of the material.
The dummy block or pressure plate, is placed at the end of the
ram in contact with the billet.
Friction is at the die and container wall requires higher
pressure than indirect extrusion.

23. Function Of Dummy Block
To repeatedly transmit the force of the ram, at high
temperature, to the alloy.
To expand quickly under load and maintain a secure
seal with the container wall.
To separate cleanly from the billet at the end of the
stroke.
To cause no gas entrapment that can result in
blistering, or damage the face of the container and/or
dummy block.
To compensate for minor press misalignment.
To be quickly and easily removed and replaced.
To function effectively until a production run is
complete.

24. In-direct Extrusion
The hollow ram containing the die is kept stationary
and the container with the billet is caused to move.
Friction at the die only (no relative movement at the
container wall) requires roughly constant pressure.
Hollow ram limits the applied load.

25. Hot Extrusion
It is done at fairly high temperatures, approximately 50 to 75% of
the melting point of the metal.
 Die life and components are effected due to the high
temperatures and pressures, which makes lubrication necessary.
Pressures Ranges: 35-700 MPa.
Hot extrusion is a hot working process, which means it is done
above the material's re-crystallization temperature to keep the
material workable hardening and to make it easier to push the
material.
The biggest disadvantage of this process is its cost for
machinery.

26. TYPES OF HOT EXTRUSION
Hot Extrusion
Lubricated
Non-Lubricated
Hydrostatic

27. sc
LUBRICATED HOT EXTRUSION
Before the billet is inserted into the hot extrusion container, a
suitable lubricating system is positioned immediately ahead of the
die in order to reduce frictional stresses.
Oil and graphite are used at lower temperature whereas at
higher temperatures, glass powder is used.
Copper alloys, titanium alloys, alloy steels, stainless steels, and
tool steels are extruded using lubrication.
Lubrication

28. NON-LUBRICATED HOT EXTRUSION
No lubrication is used on the billet, container, or die for
reducing frictional stresses.
 It has the ability to produce very complex sections
with excellent surface finishes and low dimensional
tolerances.
Solid and hollow dies with flat shear faces are typically
used.

29. HYDROSTATIC EXTRUSION
In the hydrostatic extrusion process the
billet is completely surrounded by a
pressurized liquid, except where the billet
contacts the die.
The rate, with which the billet moves
when pressing in the direction of the die, is
thus not equal to the ram speed, but is
proportional to the displaced hydrostatics
medium volume.
The process must be carried out in a
sealed cylinder to contain the hydrostatic
medium.
 Pressure = 1400 MPa (approx.)

30. HOT EXTRUSION
ADVANTAGES
Complex solid or hollow shapes can be produced.
Small quantities can be economically produced.
 Delivery times are often far shorter than alternative
processes.
DISADVANTAGES
 High equipment set up and maintenance cost.
Extrusion process for metals is at very high
temperatures.
Die is preheated to increase its life, so there are
chances of oxidation of hot billet.
Process Wastage is higher as compared to rolling.
Non-homogeneous.

31. Cold Extrusion
Cold extrusion is the process done at room temperature or
slightly elevated temperatures.
 This process can be used for materials that can withstand
the stresses created by extrusion.
 Cold extrusion is done at room temperature or near room
temperature. The advantages of this over hot extrusion are
the lack of oxidation, higher strength due to cold working,
closer
tolerances, good surface finishing.
Examples of products produced by this process are:
collapsible tubes, fire extinguisher cases, and shock absorber
cylinders.

32. COLD EXTRUSION
ADVANTAGES
Improved Mechanical
properties.
Good control of dimensional
tolerances.
Improved surface finish.
No need for heating billet.
No oxidation takes place.
DISADVANTAGES
Tooling cost is high, therefore
large production lot size is
required.
Special coating is required to
reduce friction and to maintain a
lubricant film throughout.
Limited deformation can be
obtained.

33. Application Of Cold Extrusion
Cu, Pb, Sn, Al Alloys, Ti, Mo, V,
Steel, Zr parts can be extruded.
Tubes, Gear Blanks, Aluminum
Cans,Cylinders, Fire Extinguisher
Cases, Shock Absorber Cylinders,
and Automotive Pistons are
manufactured.

34. Impact extrusion
A manufacturing process in
which a small shot of solid
material is placed in the die and is
impacted by a ram, which causes
cold flow in the material.
It may be either direct or
indirect extrusion and it is usually
performed on a high- speed
mechanical press.

35. Stripper
plate
Punch
Blank
Die
Part
Although the process is generally performed cold,
considerable heating results from the high speed
deformation.
A short lengths of hollow shapes, such as collapsible
toothpaste tubes or spray cans.
 Requires soft materials such as Al, Pb, Cu or Sn.
Applications

36. Horizontal extrusion process
 The layout of operating
machine is horizontal and
movement of billet as well
as of ram is horizontal in
direction.
 15-50 MN capacity.
 It is mostly used for
commercial extrusion of
bars and shapes.
Disadvantages:
• deformation is non-uniform
due to different temperatures
between top and
bottom parts of the billet.

37.  The movement of billet and ram is vertical in orientation.
 3-20 MN capacity.
 Mainly used in the production of thin-wall tubing.
Vertical extrusion process
Advantages:
 Easier alignment between the press
ram and tools.
Higher rate of production.
Require less floor space than horizontal
presses.
uniform deformation, due to uniform
cooling of the billet in the container.
Requirements:
 Need considerable headroom to make
extrusions of appreciable length.
A floor pit is necessary.

38. DIE MATERIALS
Dies are made from highly alloy tools
steels or ceramics (zirconia, Si3N4 ).
(for cold extrusion offering longer tool
life and reduced lubricant used, good
wear resistance).
Wall thickness as small as 0.5 mm
(on flat dies) or 0.7 mm (on hollow
dies)
can be made for aluminium extrusion.
 Heat treatments such as nitriding are
required (several times) to increase
hardness (1000-1100 Hv or 65-70 HRC).
This improves die life. avoiding
unscheduled press shutdown.
There are two general types
of extrusion dies:
1) Flat-faced dies
2) Dies with conical entrance
angle.
Ceramic extrusion dies
steel extrusion dies

39. LUBRICATION
NEED-
Proper lubrication is essential in extruding, in order to
improve die life, reduce drawing forces and
temperature, and improve surface finish.
Types of Lubrication
a)Wet : Dies and Rods are completely immersed in
lubricant.
b) Dry : Surface of the rod to be drawn is coated with a
lubricant.
c) Coating : Rod or Wire is coated with a soft metal that
acts as a solid lubricant.
d) Ultrasonic Vibration : of the dies and mandrels.

40. FACTORS AFFECTING THE EXTRUSION
FORCE
1. Type of extrusion
2. Extrusion ratio
3. Working temperature
4. Deformation
5. Frictional conditions at the die and
the container wall

41. Extrusion: was originally applied to the making of lead
pipe and later to the lead sheathing on electrical cable.
Extrusion of lead sheath
on electrical cable.