In order to make a good product using the

extrusion process it is of utmost

importance to have a well homogenised

melt delivered to the die (blown, cast, pipe,

profile etc.)

The homogenous melt is the key to

dimensional stability, process stability,

uniform texture, and for good mechanical

properties of any extruded product like

film, sheet, pipe, profile, etc.

For extrusion the polymer mixing is a two step process:

Dry Mixing: mixing of different polymers (in granular

or powder form), master batches and additives before

the mix is introduced to the extruder. This can be done

either offline or online with the help of sophisticated

dosing systems (volumetric or gravimetric type).

Wet Mixing: The final mixing takes place inside the

extruder when the polymer blend is melted inside the

barrel with the help of external (heater bands) and

frictional heat (screw rotation). Various types of

mixing elements or components are used for the

homogenous mixing of the polymer melt inside the

extruder.

The most common dry mixing methods are volumetric mixing and

gravimetric mixing. This can be batch mixing or continuous mixing

(loss in weight).

In batch mixing various polymers and additives are stored in

different hoppers and a pre set amount of each component is filled

in a mixing hopper, once the batch is completed the mixer motor

started and the blend is prepared.

In continuous/loss in weight mixing all the components are feeded

simultaneously in to the feed throat of the barrel as per the set

percentage.

Another good example of dry mixing is the preparation of PVC

compound. This is a batch mixing process where all the ingredients

(PVC resin in powder form and liquids like plasticizers, stabilizers

etc.) are poured in a high speed mixer using a certain method and

then this compound (almost dry) is cool down in a cooling mixer

prior to be introduced to the extruder.

Wet mixing takes place inside the extruder

when all the ingredients start melting.

In the smooth bore extruders, it starts when

the polymer is melted.

In grooved feed extruder, it starts right at the

grooved feed section where pellets are

chopped by the grooves.

Various mixing techniques, components are

used to get the homogenous mix.

We will discuss wet mixing in detail but before

that let’s talk about the fundamentals of

extrusion process and the equipment involved.

Extrusion: is a continuous process in

which the plastic is melted, mixed,

homogenized and pushed out of a die.

Co-Extrusion: When two or more extruders

are used to feed the die than it’s called Co-

extrusion process. It can be used in Blown,

Cast, Pipe, profile or Sheet applications.

Single Screw Extruder: Only one screw is used to convey, melt and

plasticize the polymer.

Twin Screw Extruder: In this extruder the barrel is in the design of

horizontal No.8 in which two screws are either inserted to be in co-

rotation or counter rotation arrangement.

Vented Barrel Extruder: In this extruder , the barrel has an

opening where the vacuum is applied to extract the fumes, gases

etc. This arrangement can be applied in single screw or twin screw

extruders.

Planetary Extruder: This is a very special design where two

extruders are connected to complete the job of homogenizing the

polymer melt. The first extruder is a type of conveyor with pre

melting and the second extruder is the real mixer where mostly

shear heat is used to melt and homogenize the polymer. It contains

a center screw with 6, 8 or 10 counter rotating satellite screws

embedded in the core screw. This is commonly used for PVC where

both shear heat and short residence time are crucial factors.

The typical components of an extruder:

Electric Motor

Reduction Gear Box with thrust bearing

Flange

Coupling

Cooling Jacket for Feed throat

Smooth/Grooved Feed Section

Barrel

Screw

Heating and Cooling options – Band Heater and Cooling

Blowers, Water cooled heaters

Screen Changer

Breaker Plate and Screens

Adaptor

Melt Pressure Transducer

Rupture disc

Melt Temperature Probe

Barrel is a strong hollow pipe made out

of steel the bore of which is hardened

either by Nitriding or a bi-metallic liner

is used.

On one end Barrel is attached to the

gear box whereas screen changer is

usually connected to the other end. The

barrel is levelled and supported using

the brackets to prevent bending.

Bi-metallic Barrel: Is made out of micro-

alloy (Nickel-rich iron-boron alloy) steel

with a wear and corrosion resistant

alloy lining. This is a straight seamless

pipe in which the screw rotates. The

barrel provides the controlled

temperature to heat the polymer inside.

The heaters can be air cooled or water

cooled depending upon the resin being

processed. Water cooling is preferred

for heat sensitive polymers.

Screw is the most critical component of the

extruder. It is the screw which provides the

homogenized melt to the die. Screws can be

general purpose or specific.

General purpose screws are mostly used in case

of co-extrusion where various polymers are

processed using the same extruder depending

upon the film structure.

The screw can be of :-

Conventional design:

Feed,

Compression and

Mixing/Metering Section

Barrier design:

Feed-it can be single flight or double

flight,

Barrier flight with or without

compression,

Metering section with one or multiple

Mixers.

Feed section

Beginning of barrier flight

End of barrier flight and beginning of mixing section

Feed Throat: This is the

opening where dry mixed

polymers are introduced to

the extruder. Feed throat

can be directly cut in to the

barrel (built in) or it can be

provided in an additional

part (feed box and feed

sleeve) connected to the

barrel. Feed throat is

normally kept cold (except

in the case of PA where it is

kept warm) to prevent the

resin from melting in the

feeding area, otherwise a

solid polymer ring (also

called bridging) will form

around the screw and will

not allow the resin to move

forward.

In built feed throat

Add on feed throat

The feed area can be smooth finish or

with linear grooves. In North America

smooth bore is commonly used whereas

in Europe most of the extruders are

designed with grooved feed section. The

linear slots can be rectangular or V-

shape. Also the number of slots can vary

with different machine manufacturers.

The main advantage of using grooved

feed section is the improvement in

throughput.

Polymer slippage at the feed throat is a

big problem small diameter (1”-1.5”)

smooth feed extruders that can be

corrected by either using shallow

grooves in the barrel or by using a

proper grooved feed section.

Feeder sleeve

Grooved feed

Smooth bore extruders have positive pressure gradient

towards the tip of the screw. The pressure inside the

barrel builds gradually and is maximum at the tip of

the screw. The screw acts as a pump and the pre-mix

dry blend is conveyed like a displacement pump. All

the ingredients starts melting one at a time (depending

upon their melting temperature) using shear heat as

well as external heat provided by the barrel heaters.

The polymers/additives with the lowest melt

temperature will melt first followed by the higher melt

temperature polymers. Most of the mixing takes place

after the compression section when all the polymers

are fully melted, the pressure is slightly dropped once

the melt passed through the mixing sections of the

screw. Up to certain extent the throughput of these

extruders is directly affected by the melt pressure.

Old design grooved bore extruders are negative pressure extruders

where the pressure is maximum at the feed section and gradually

drops till the tip of the screw. The above condition was a draw

back (high torque required, high wear and tear, poor mixing) for

the grooved feed extruders which has been taken care in the newer

extruders with the help of screw design.

In newer grooved feed extruders , even the pressure is still

maximum at the feed section but it is only for a short period (2-3 D

only) after that the screw has a decompression section which helps

in reducing the pressure. After this decompression section the

screw is similar to the smooth bore screw but slightly flat (less

C/R). The screw works as a positive pressure screw with positive

pressure at the tip of the screw. The new generation screws for the

grooved feed extruders have taken care of the torque. The mixing

has also been improved but still it’s not as good as in the smooth

bore extruders so various mixing sections are added in the screw

design.

Maddock (Union Carbide)-

Dispersive Mixing

Spiral Mixing Section –

Dispersive Mixing

Blister Rings –

Dispersive/Distributive

Mixing

Pineapple mixing –

Distributive Mixing

Mixing Pins – Distributive

Mixing

This mixer is one of the most commonly used

mixers in North America. This mixer was originally

developed by Union Carbide lab in New Jersey. Over

a period many new versions specially twisted

Maddock were developed. Twisted Maddock mixers

have a very complex design with many variables so

they can not be generalised for all the polymers.

Therefore, straight Maddock is still the most

preferable design and is commonly used. This mixer

is dispersive type but some distributive mixing also

takes place. The most critical dimensions for this

mixer are:

Clearance of the barrier flight: lower the

clearance, high shear, more frictional heat,

better mixing, high pressure drop.

Barrier flight width: wider flight means high

shear, high frictional heat, high pressure drop

and better mixing.

Total length of the mixing section: Long mixing

section will lead in high pressure drop, more

frictional heat, (common length is 2-2.5D),

possible degradation.

Channel depth: must be calculated on the basis

of throughput. Too shallow will reduce the

throughput, if too deep than polymer can hang

and degrade, long purging time.

This mixer is considered

quite aggressive and

provides both distributive

as well as dispersive

mixing. The melt is

divided in to several melt

streams as it passes

through the criss-cross

holes of the mixer rings.

Though this mixer

provides good mixing but

at the same time the

pressure drop is quite

high along with high

frictional heat. The wear

and tear is quite high.

Pin mixers have been used

in the extrusion screws

since long time. Pins can be

mounted in various

patterns. They always

restrict the flow and the

shear heat is generated

along with the pressure

drop which depends upon

the number of pins and the

covered area. There is a

draw back in this design as

dead spots occur behind the

pins. These kind of mixers

works fairly well and

provide good distributive

mixing but the performance

varies by the type of

polymer being processed.

These mixers are

usually in-built almost

at the tip of the screw

and are commonly

used along with the

Maddock mixer or

another high shear

mixers for improved

dispersive/distributive

mixing. If designed

properly they work

fairly well.

Screen Changer: The function of

the screen changer is to remove

the foreign particles from the melt

with the help of breaker plate and

screens/mesh.

Breaker Plate and Screens/mesh:

Breaker plate is a cylindrical plate

with holes and is a removable part

of the screen changer. Breaker

plate supports the screen/mesh.

The screens/mesh are made with

woven wires and comes in various

sizes.

Apart from preventing the

foreign particles passing

through the die the breaker

plate and screen pack assembly

also serves to create back

pressure in the barrel. Back

pressure is required for uniform

melting and proper mixing of

the polymer.

The pressure transducer is

normally mounted at the end of

the barrel and measures the melt

pressure exerted on the barrel

wall. The pressure is a good

controlling factor in the extrusion

process and the probe must

always be in working order.

Melt Pressure is a good diagnostic

factor in extrusion process, a

sudden increase in pressure

indicates blockage due to foreign

particles, un-melted polymer etc.,

a sudden drop in pressure

indicates empty hopper or some

blockage at the feed throat

(bridging) etc. A gradual increase

in pressure over a period of time is

a good indicator for the operator

that screens are choked and must

be replaced with clean ones.

This is the safety plug for

the extruder. It is mounted

at the exit side of the barrel.

There are different pressure

rated rupture discs

available. The selection of

rupture disc depends upon

the downstream hot parts

(die) pressure bearing

capacity. If for any reason

the pressure goes higher

than the disc pressure the

disc membrane will rupture

and the polymer will start

leaking.

The melt temperature probe

comes in direct contact with

the polymer melt and

measures the actual melt

temperature. It can be installed

in the barrel or in the adaptor.

Variable depth melt probes are

used to check the temperature

difference, ∆T of the melt

stream inside the adaptor. The

probe tip can be moved in and

out to measure the melt

temperature at various depths

in the melt stream. The

uniformity (within +/- 5 °C ) of

the melt temperature close to

the adaptor wall and in the

center of the melt stream is an

indicator of good mixing. If the

difference in melt temperature

is over +/- 5 °C than efforts

must be made to resolve this

issue.

Variable Depth Melt Probe

Fixed and variable depth melt probes

Static Mixers: If for some reason the melt is

not homogenized properly by the screw

than the melt quality can be improved

inside the adaptor by using certain static

mixers. There are various designs available

of which Sulzer mixer is quite common. The

mixer is installed inside the adaptor bore

and helps in bisecting/mixing the melt.

If the temperature difference (∆T) inside

the adaptor is too much (measured using

variable depth thermocouple then these

mixers help in bringing the melt at a

uniform temperature before it enters the

die. Even though these mixers do not fix

the root cause of the problem (mostly poor

screw design) still they are helpful in

improving the melt quality

Note: Static mixers should not

be used for thermal sensitive

polymers like PVC, EVOH,

PVdC etc.

Feed section Beginning of barrier flight

Melt separation End of barrier flight

Fully melted , just before the mixing section

With over 20 yrs. Of experience in the field of plastic extrusion

machinery, Mr. Ajay Beniwal has gained knowledge through hands on

practical experience. What ever information is shared in this

presentation or any other previous presentations is the collection of

hard work and dedication towards his work in the field of plastic

extrusion.

Extru-tech Solutions is a consulting and service providing company in

the field of plastic extrusion with specialization in Blown film, Cast

film, Sheet, Double/triple-Bubble process, PVC cling film process, EVA

sheet (PV application), PVB Sheet process for windshield and

architectural application, and various Coating/Lamination processes.

We also provide consulting services to the equipment manufacturers

interested in developing any of the above mentioned technologies.

We do provide training to Supervisors and Operators

at very reasonable cost.

Phone: +1-647-687-0859

Email: beniwal@extru-techsolutions.com

EXTRU-TECH SOLUTIONS INC. 11 SEASCAPE CRESCENT, BRAMPTON, ON, L6P3C4

www.extru-techsolutions.com