Pipe extrusion line is important for pipe processing

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Engineering and Technology, INDIA
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PIPE EXTRUSION

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Engineering and Technology, INDIA
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Pipe plant elements
• Extruder
• Die
• Sizing device
• Cooling bath
• Puller
• Cutter or Winder

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Engineering and Technology, INDIA
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Pipe Extrusion Plant

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Engineering and Technology, INDIA
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MATERIALS
• PVC
• HDPE
• PS
• ABS

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Engineering and Technology, INDIA
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PVC Pipes are available in
two forms:
• Rigid PVC or Unplasticised PVC
• Soft PVC or Plasticised PVC

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Engineering and Technology, INDIA
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PVC can’t be processed
alone. It needs to be
compounded by adding
different ingredients.

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Engineering and Technology, INDIA
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Ingredients added in PVC
compounding
• Plasticizers
• Fillers
• Heat stabilizers
• Lubricants
• Processing aids
• Flame retardants
• Colorants
• Impact modifiers

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Engineering and Technology, INDIA
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Plasticisers are used to make
the resin softer, reduce the
viscosity and thus make
processing of PVC easier. Di-
allyl phthalate (DAP) and
di-2-ethyl hexyl phthalate
(DOP) are examples of
common PVC plasticisers.

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Engineering and Technology, INDIA
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Heat stabilizers are required to
prevent the thermal degradation of
PVC at the processing temperatures.
The degradation involves resin
discoloration and evolution of HCl. In
the past lead stabilisers have been
used (viz. lead stearates). Owing to
toxicity of Pb, today the most
common stabilisers are based on tin,
viz. di-butyl tin maleate.

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RPVC-pipe formulation
PVC Resin (K = 67.68) 100 parts
Tin stabiliser 0.4
Calcium stearate 0.8
Wax 1.2
TiO2 (colorant) 1.0
CaCO3 20 to 40

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Lubricants are used to reduce
friction during processing that may
lead to degradation. Two types of
lubricants are used: external and
internal. The former (viz. calcium
stearate and wax) is used to reduce
friction between metal surfaces and
the polymer, the latter (viz. stearic
acid) the friction arising out of
polymer chain slippage.

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Fillers and reinforcements are added
for cost reduction (fillers) and
improvement of the mechanical
properties (reinforcements). Both
increase rigidity, hardness, density and
fire retardancy. A typical filler is
calcium carbonate; typical
reinforcements are chopped glass fibre
and mica flakes, while such additives as
clay, talc, and silica are in-between.

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Flame retarding agents are
needed for plasticised PVC.
Chlorinated paraffin oil or
wax, oxides of Sb, Mo, P,
or B, as well as aluminium
hydroxide have been used.

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Engineering and Technology, INDIA
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Processing aids are added
to rigid PVC to improve
flow through the barrel
and die. To this group of
additives belong polymers
and copolymers miscible
with PVC, e.g., PMMA,
ABS, SAN.

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Impact modifiers are added to
improve the impact strength.
Impact modifiers are composed
of at least two parts: one that
ascertains miscibility with PVC,
the other elastomeric in nature
that provides the toughening
effect. Examples of these
include MBS, EVAc, CSR, and
NBR.

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PVC compounding is usually
carried out in two stages:
• Dry-blending PVC resin with additives in
an intensive dry mixer, equipped with a
high speed impeller.
• Melt blending in a suitable compounder,
e.g., single-, twin-, or multi-screw
extruders, single or twin shaft
compounders, Kinetic energy mixer, etc.

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Sizing equipments
The extrudate emerging from the
pipe die usually shows variation of
diameter, caused by variability of
the rheological melt properties.
Thus, the pipe has to be
calibrated or “sized” while hot.

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Engineering and Technology, INDIA
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Four methods are in use for
pipe sizing
• Vacuum trough
• Sizing sleeve
• Extended mandrel
• Sizing plates

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Vacuum trough
The pipe, coming out of the die passes
through a long closed trough filled with
cooling water. Inside the trough, the
pipe passes through fixed metal rings
of predetermined size. Since, vacuum
is drawn over the water, the internal
pressure in the soft pipe causes it to
expand against the rings. Thus the
pipe gets the outside diameter.

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Sizing sleeve
This method fixes the outside pipe
diameter as it hardens by contact with
a water-cooled metal sleeve. The
contact is achieved by air pressure
inside the pipe or by drawing vacuum
through the perforated inside surface
of the sleeve. The method produces
glossy, smooth, closely controlled outer
surface.

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Extended mandrel
This method uses a water-cooled
metal mandrel that extends
beyond the die. Shrinkage of pipe
as it cools helps to keep a light
contact with the mandrel. Thus
this method provides additional
internal cooling and internal
support.

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Engineering and Technology, INDIA
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Sizing plates
This method involves pulling the
pipe through a series of brass or
stainless steel plates. Air pressure
in the pipe forces it against the
plates, thus controlling the outside
pipe diameter. This method is
mainly used for smaller diameter
pipes.

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Cooling water troughs
• During the sizing also the pipe gets
cooled.
• Cooling is done by spraying water over
the surface of the pipe.
• The length of the cooling troughs for PE
is longer than for PVC.

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Pulling Machines
• Pulling machines maintain uniform pipe
size.
• Pullers are either the caterpillar type or
the gripping wheel type.
• The puller speed is controlled by
electrical, electronic or magnetic speed
variators.

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Controls
• They provide automatic measurements and
control of pipe size.
• In one system the capacitance of the pipe is
measured and is related to the thickness.
• The sensing head moves around the pipe,
showing when the pipe is out of round and
when it is generally too heavy or too light.
• All these systems can be made to feed back
to control the puller or the extruder drive.

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Cutting devices
• Circular saw cutters, with blades specially
designed for cutting plastics, are used to cut
the pipe into specified lengths.
• Automatic cutting devices are also used.
They travel with the pipe to cut, then after
cutting go back to pick up the next pipe
length.
• Larger sizes are sawed to length.

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Coiling devices
• Smaller diameter Soft PVC and HDPE
pipes are coiled on a large centre-wood
reel.
• In simple methods pipe is coiled by a
band rank.