2. EXTRUSION PRINCIPLES
Introduction
Extrusion of thermoplastics is a process in which the material is melted by external heat /
frictional heat and conveyed forward by a screw to the opening of the die, which gives
the shape of the required product. Extrusion process is a continuous process by which
many products like Films, Raffia tapes, Pipes, Sheets, Mono filaments, Fibre and
Filaments can be manufactured.
Extruders
Extruders comprise of Hopper, Barrel/Screw and Dies. Figure 1 shows the components
of a modern extruder
Hopper
All the extruders have an opening in the barrel wall at the driven end, through which the
plastic granules enter the extruder. The hopper, a simple sheet - metal enclosure, is
mounted above that opening and holds about an hour's capacity material. Hopper is
provided with heating system, if the material has to be preheated before entering the
extruder.
3. 3
Screw
This is the heart of the extruder. Screw conveys the molten polymer to the opening of the
die after properly homogenising the molten polymer. Figure 2 illustrates a typical screw
configuration.
There is considerable variation in the design of the screw for various materials, the most
important variable being the depth of the channels. Despite much desire for universal
screw, it is advisable to use a different design for each material to achieve the best results.
So for optimum process of PVC, PP/PE following screw designs are advised.
PVC Screws
Since PVC is thermally unstable material, the screw (Fig 3) for processing this material
has deeper channels, little or no metering, hard chrome plated, conical nose to prevent
material hangup. Screw diameters vary from 30mm to 140mm, L/D ratios vary from 18
to 22 for single screw and from 16 to 18 for twin screw extruders. Compression ratios
are between 1.5 to 2.2:1 for twin screw / single screw extruders. Venting unit is present
on the extruder to remove any volatiles.
4. 4
PE/PP Screws
PE/PP screws (Fig. 4) are more or less similar. The screws are designed to have shallow
channels, sudden compression and long metering zones. Screw L/D varies from 24:1 to
33:1 and the diameters from 20 mm to 250 mm and the compression ratios from 2.5 to
3.1
Barrier - Type Screws
In some cases, even an optimally designed screw is not capable of completing the melting
process. In such cases, the extrudate will contain unmelted plastic. This can be
prevented by building a barrier or secondary flight in the channel (Fig. 5). The barrier
flight is undercut and permits the passage of only fully molten plastic. The design of the
barrier screw ensures complete melting of the polymer solids.
There are different designs of barrier screws now available i.e. Maillefer / Uniroyal,
Hartig, Barr I and II etc, each with its own characteristics.
5. 5
Mixing Heads
The metering section of a standard screw is not a good mixer. Smooth laminar flow
patterns are established in the channel which do not mix dissimilar elements in the melt.
Mixing devices are frequently installed in screws to distrupt these flow patterns and
improve melt homogenisation.
Pin mixers (Fig. 6) / Du Pont
mixer are the examples of
mixers which operates with low
shears. They operate by
interrupting and recombining the
flow many times. They can be
installed easily on existing
screws to improve the
performance.
Other type of mixers are Maddock
(Union Carbide) (Fig. 7) and Egan
(Fig. 8). These mixers operate
subjecting the melt to high shearing
forces. Maddock screw operates like
a barrier type of screw, it separates
out any unmolten particles and
homogenise the melt. Egan mixer
also operates like a barrier screw,
helical splines contribute to forward
flow and helps in homogenising the
melt.
6. 6
Breaker Plate / Screen Pack
Breaker plate with screen packs (Fig. 9) inserted are kept in the adapter which is a joint
between the end of the extruder and the beginning of the die. This assembly has several
functions
• Arrest the rotational flow of the melt and convert into axial flow
• Improves melt homogenity by splitting and recombining the flow
• Improves mixing by increasing back pressure
• Removes any contaminants and unmelt
Screen packs are made up of series of screens of different mesh, with the coarest screen
placed against the breaker plate to support the finer screens.
Dies
Various types of dies are used for processing PVC and PE/PP. They can be either
Annular dies or Flat dies (T-Die or Coat hanger dies). Details of various dies are given in
Anneure - 1
Annular Dies
PVC Dies
PVC is thermally unstable, so it
requires perfect flow channels. A
spider mandrel (Fig. 10) die is used
which helps in dividing the melt and
fulfils the task of better
homogenisation and allows the melt to
flow smoothly.
7. 7
PE/PP Dies
For processing PE/PP a die with spirals are
used as illustrated in Fig. 11. As the plastic
flows from the entry point it spirals around
the mandrel section of the die. From the
Fig. 10, it is clear that the land depth
between the spiral section and the wall
increases as the material progresses through
the die. As a result, the distribution around
the die periphery is made uniform in order to
control the gauge of the extruded tube.
Flat Dies
There are two types of Flat Dies. One is T-Type of flat die (Fig. 12) and the second one
is Coat - Hanger type of die (Fig. 13). In coat hanger type of die, the design is of
triangular preland section, which gives balanced pressure leading to uniform flow of the
material. This minimizes the adjustment required to obtain uniform sheet thickness.
8. 8
Auxiliary Equipment
Once the product in the molten form (Films, Sheets, Pipes, Fiber setc., ) comes out of the
die, it is cooled by chilled water/air/rollers etc., and wound/cut by various means. In
some process like BOPP, Raffia tapes, Fibre etc. after cooling initially they are reheated
by passing them through ovens and stretched to obtain required mechanical and optical
properties. Various down stream equipment used in different process are given briefly in
the following table.
Product Process Quenching Stretching Stabilisatio
n
Winding /
Cutting
BOPP Tenter frame Chilled roll/
Chilled
water
MDO/TDO
Hot air Oven
Oven Rolls
Double
Bubble
Chill water Nip rollers
and inflating
the bubble
Air Rolls
TQPP Inverted
bubble
Chill water Nip rollers
and inflating
the bubble
Air Rolls
Cast Flat film Chill rolls Rollers Air Rolls
Raffia Flat film
then cut into
tapes
Water Goddet
rollers and
Hot air oven
/ hot plate
Oven Bobbins
F&F Spinning Chill air Goddets and
hot plate /
hot rollers
Air Bobbins
Sheet Flat Rollers -- Air Rolls / Cut
leng.
Pipes -- Vacuum
sizing and
chill water
-- Water Cut leng.
9. 9
ANNEXURE 1
VARIOUS DIES WITH DIFFERENT DIE GAPS FOR PP/PE/PVC APPLICATIONS
SECTOR POLYMER APPLICATION TYPE OF DIE DIE GAP
(MM)
REMARKS
Film Polypropylene BOPP - Flat Coat hanger 2 - 2.5
- Blown Spiral mandrel 1.75 - 2 Double Bubble
TQPP - Blown Spiral mandrel 1 - 1.5 Downward Extrusion
Cast - Flat Coat hanger 0.8 - 1.2
Polyethylene LLDPE - Blown Spiral mandrel 1.8 - 2.5
- Cast Coat hanger 1.5 - 1.8
LDPE - Blown Spiral mandrel 0.8 - 1
HDPE - Blown Spiral mandrel 0.8 - 1
PVC PVC - Blown Spider mandrel 0.8 - 1
Raffia Polypropylene PP - Flat Coat hanger 0.6 - 0.7 Die widths can be adjusted
with deckles
Polyethylene PE - Flat Coat hanger 0.7 - 0.8 -do-
Extrusion Coating Polypropylene PP - Flat Coat hanger 0.5 - 0.6 -do-
Polyethylene PE - Flat Coat hanger 0.6 - 0.7 -do-
Sheet Polypropylene Sheet - Flat Coat hanger 0.8 - 10 Die manifolds can be changed
and make sheets from 0.750
mm to 8 mm thick sheets
Coat hanger 0.3 - 1 For making thinner gauge
sheets / films
Pipes Polyethylene/
Polypropylene
Small bore tubings /
Pipes
Spiral mandrel 0.25 - >5 Depending on the class of the
pipe thickness varies
PVC Pipes Spider mandrel >1 Depending on the class of the
pipe thickness varies
