1Polymer Processing (CH-511)

2Learning Outcome

Develop an appreciation of the extrusion technology

and mechanics of melt preparation.

Reading List:

Plastics Engineering: R.J. Crawford Pergamon, 1981

Plastics Material & Processing : A.B Strong,

Modern Plastic Handbook: Charles A. Harper

3Extrusion

Extrusion is a process in which solid thermoplastic material is

melted and forced through an orifice (Die) of desired shape and

then cooled.

Extruded products:

Sheet Film Tube Profile (e.g. PVC window frames) Cable coating

Process based upon screw extrusion:

Injection moulding

Injection blow moulding Extrusion blow moulding

4This section covers the basics of extrusion. There are many

products made via extrusion and most of them involve

machinery for melting the plastic (an extruder) and the means to

shape the plastic melt (a die).

So the processing steps involved in extrusion are :

Feeding (Add the resin).

Melting ( add heat to the polymer).

Pumping (transport of the melt).This can be accomplished by generating melt at a high pressure

or by using an actual pump (like a gear pump)

Forming ( shaping the melt ).

Cooling (remove excess heat so that the material becomesrigid and retains its shape ).

5Sketch of Extrusion Line

Thus the function of the extruder is to :

- melt the plastic

- pump the plastic

6Types Of Extruder

1. Single Screw Extruder.

2. Twin Screw Extruder.

A single-screw extruder consists of a screw in a metal cylinder or

barrel. One end of the barrel is attached to the feed throat while

the other end is open.

Single Screw Extruder

7A. Screw

B. Barrel

C. Heater

D. Thermocouple

E. Feed Throat

F. Hopper

G. Thrust bearing

H. Gear reducer

I. Motor

Parts Of an Extruder

8 Hopper for feeding in materials.

A barrel assembly (usually steel)With a screw inside (used to push the plastic down the barrel, this

also generates the melt pressure)

A heating system consisting of- Electric band heaters

- Thermocouples to monitor barrel temperatures

A drive system for the screw- Electric motor

- Gear reducing assembly

A die for shaping the product

Parts Of an Extruder

9 Resin is usually fed into a hopper, a large funnel shapedcontainer which sits on top of the extruder.

Resin flows into the extruder by gravity but feed screwscan be used to control the flow rate of resin into thesystem.

Sometimes there are screen or magnets in the hopper toremove metals and other contaminants.

Vibration of the hopper walls can be used to prevent solidbridging in the throat (the smallest section) of the hopper.

Feeding System

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Hopper / throat area is cooled to prevent material from becoming sticky and blocking the channel before the material

feeds into the screw.

There are usually 3 to 5 heating zones along the barrel.

Electric heaters are important for startup but after the process is running, heat generated by friction of the melt in the barrel

supplies most of the heat (80% to 90%).

It is important to control the melt temperature for the following

reasons :

Avoid thermal degradation Too high a temperature may make post extrusion processing difficult

Heating and Cooling System

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Extrusion Screws

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Screw Design

Very small clearance between the screw flight and barrel.

Depth of flow channel usually small compared to the screw diameter.

Depth may vary along the channel.

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Screw Design

Barrel

Screw root

FeedMetering

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Analysis of Extruder Behavior

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Solid Bed

Mostly solid pellets.

Friction between the pellets and barrel causes some pellets to melt and form a film.

When enough has melted, we start to see the formation of a melt pool.

Melting

Solid particles form a solid plug which spirals along the screw

Melting occurs at the interface between solid and melt film

Melt is pushed against back flight

Inefficient melting can cause solid bed break-up

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Occurs along last few screw diameters.

All polymer should be molten.

Needs to provide stable flow to die.

Metering

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Die forms the shape of the extrudate.

Exit region called the land region.

Distributes flow exiting with uniform velocity.

Breaker plates + filters often used at die inlet.

Should allow for die swell.

Dies can be very complex (e.g. profile, spiral mandrel).

Co-extrusion dies for multiple layers.

Forming

The Single Screw Plasticating

Extruder

Pellets, powders or flakes are drawn by gravity from a hopper to the gap between a rotating screw and a heated barrel. They are transported forward, compacted, melted and pumped through a die where the melt is shaped prior to solidification by cooling.

Extruder Screw:The Extruder Screw has several important functions to perform

1. Conveying the resin to the extruder.

2. Mixing and melting the ingredient.

3. Building pressure in the screw so that the molten resin will be

pushed through the die.

L/D Ratio:An important parameter associated with the screw. It is the ratio of

length of flighted portion of the screw to inside diameter of the barrel

or outside diameter of the screw flights.

Breaker Plate:Breaker plate acts as seal between Extruder barrel and the die, it

prevent leakage of the melt.

They are fitted to support

1. Support screen pack

2. Create back pressure in the extruder.

3. Converts the rotational motion of the melt to axial motion

4. Filter foreign particles from the material.

Types of Breaker Plate1. Sliding plate.

2. Auto screen and rotary screen changer.

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Breaker Plate:

A sliding-plate screen changer has two breaker plates mounted on

a support plate; a similar screen changer uses cartridge filters in

place of the breaker plates. The breaker plates are manually or

hydraulically switched when the pressure drop across the breaker

plate exceeds a preset value. Then the dirty breaker plate and

screens are replaced.

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In an auto screen system, a steel screen is held against a fixed

breaker plate. When the pressure drop exceeds the preset value,

heaters melt the polymer at the dirty end of the screen and the

screen, which is under tension, advances. With a clean screen in

the melt stream, the heaters turn off and the polymer at the dirty

end of the screen solidifies, preventing further screen movement.

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In the rotary screen is a wheel containing a series of crescent

shaped breaker plates. The wheel is moved in response to preset

pressure limits and dirty breaker plates are replaced by fresh ones.

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DIE: Die is a melt flow channel or restriction. It is located at the other

end of the screw, so that the screw out put can be passed through

it. It is used to achieve specific cross sectional shape from polymer

melt.

TYPES OF DIE:1. Spider Mandrel Die

2. T-Die

3. Fish Tail Die

4. Coat hanger die

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Flow Behaviour of Polymer

In the metering zone of an extruder, the polymer is usuallymolten.

The rotating screw pushes the material along the walls of thestationary barrel creating drag flow (QD). This drag flow provide

forward conveying action of the extruder in the absence of die

effectively the only flow.

The addition of the die restricts the open discharge at the end ofan extruder and produces large pressure gradient along the

extruder. Since the pressure is greatest just before the die , this

head pressure creates two other flows, Pressure Flow (QP) and

leakage Flow(QL). Since they both counter the forward motion of

the melt so pressure flow and leakage flow are often lumped

together as Back Flow.

SIZING UNIT:Sizing unit determines the final dimension of the product. To assist

in defining the shape of the part during the cooling process, plates

or ring with holes of proper sizes and shapes can be placed inside

the cooling bath, so that the extrudate passing through them.

COOLING UNIT:The cooler unit reduces the temperature of the hot extrudate to

allow it to solidify and retain its sized shape. A water bath or spray

at room temperature may be used.

Haul Off Unit:The Job of the haul off unit is to pull the extrudate through the sizing

and cooling units and feed it to the cutter

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Sizing and Haul-

off Unit

Pipe Extrusion Process

Twin Screw Extruder:

In Twin screw extruder the relative motion of the flight of one screw

inside the channel of other acts as paddle that pushes the material from

screw to screw and from flight to flight

There are two different patterns for intermeshing. In one pattern the

screw rotates in the same direction i.e. both turn clockwise or both turn

counter clock wise. This pattern is called co-rotating .

In the other intermeshing pattern the screw rotates counter to each

other i.e. one rotates clock wise and other rotates counter clock wise.

This pattern is called counter rotating.

Twin screw extruder have more positive pumping action than single

screw extruder and can be used more effectively in high output

situation.

In wire coating metal wire is covered with plastic insulation.

This process requires a pay-off (input) drum, input capstan, preheater,

extruder, cross-head or offset die, cooling trough, spark tester, diameter and

eccentricity gauges, output capstan, and wind-up drum.

The pay-off drum and input capstan unwind wire at high speeds and

provide constant speed and tension.

In the preheating station, a gas burner or electrical resistance heater

preheats the wire to improve adhesion, reduce plastic shrinkage, and

remove any moisture or wiredrawing lubricant from the wire. As the wire

passes through the die, it is coated with polymer melt.

The coated wire is cooled in a series of water baths and then the coating

is measured and tested prior to being wound up on the wind-up drum.

Wire Coating

Melt Fracture:

Extrusion is essentially the process in which a plastic wire or sheet is

made by pumping a polymer melt through a pipe or slit, and having it

solidify.

One of the basic problems that polymer industry is facing since long

is that when the polymer exits too fast from an extruder, the surface

starts to exhibits irregularities.

For slow flow rates ,the flow is smooth, As soon as the flow rate

increases the irregularities progressively get stronger.. At even higher

flow rates, the distortions can become so severe that they cause the

extrudate to break --- hence the name melt fracture.

Five pictures of a of molten polyethylene flowing out of a pipe, visible at the top.

The flow rate increases from left to right. Note that in the two leftmost

photographs the extrudates are nice and smooth, while in the middle one

irregularities start to develop. As the flow rate increases even further towards

the right, the amplitude of the irregularities gets stronger. When the flow rate is

enhanced even more, the extrudate can break. Hence the name melt fracture

Die Swell:

Die Swell occurs when an extrudate has larger diameter than the die. It

is caused by the releases of stresses in the extrudate leaving a

contraction the extrudate remembers its original shape and attempts toreturn to it. Die Swell can result in an extrudate with several time the

diameter of the die, depending on the process and condition.

Calendering:

In the calendering process the extrude is extruded directly into the nip

area between two rolls. The gap between these two rolls is small and

therefore the plastic material builds a small pool of material, this pool of

material is called extrudate bank.

The rolls have small gap between them and the plastic is forced through

this gap by the counter rotation of the rolls. The gap sets the initial

thickness of the sheet material.

The stack roll define the thickness of the film and sheet by adjusting the

gap and relative rotational speed.

These films and sheets pass through cooling rollers to set the final

dimension of the part.

Finally the material moves to a take off station usually roll up.

Types Of Rollers

I-Type

Z-

TypeL-

TypeF-

Type

Inverted-

Type