98 en Single Screw Extruders

7/31/2019 98 en Single Screw Extruders

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Sing le-Screw Ext ruders and Bar r ie r Screw s1

Peter Fischer, Johannes Wortberg

1 Extended version of a paper presented at the VDI conference on "The Single Screw Extruder Basics and

System Optimization", published by VDI-Verlag Dsseldorf, 1997 Kunststofftechnik


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The development of

s ing le -sc rew p las t i f i ca t ing

extruders

In the USA, extruder

development was - and still is -

largely characterized by

machines with smooth barrels.

Further development has tended

to concentrate more on the

screws than anything else, with

so-called 'barrier screws'

screws in which the solid

material is kept separate from

the melt in the melting section

at the center of attention.

Although the first barrier screw

was actually invented in Europe

in 1959 by Maillefer, most of the

further development work and

the practical application of this

principle took place in the USA.

The first USA patent was not

applied for until 1961 by Geyer

from Uniroyal [1].

Even today, smooth-bore

extruders with barrier screws are

superior to grooved barrel

extruders for many applications,

provided the conveying stability

is adequate. This applies in

particular to applications in

which fluctuating proportions of

recycled or regrind material

have a disruptive influence on

the normal conveying

characteristics of the solid

material. In such cases,

extrusion is likely to be more

stable with a smooth-bore

extruder.

In Europe, the development of

extruders with heat-separated

grooved bushes in the feed

section began at the end of the

fifties and beginning of the

sixties. Grooves in the barrels to

increase barrel friction and

assist conveying of the solid

material had been tried out long

before then. They were,

however, not enough to process

the newer high-molecular weight

HDPEs in powder and grit form.

This specifically European

phenomenon on the raw

materials side has come from

the systematic analysis and

development of the grooved

bush principle.

Extruders with grooved bushes

were initially operated with the

conventionally flighted three-

section screws commonly used

in Europe. To have better

control of the melt temperatures,

vented screws were later

developed, and, to improve the

melt homogeneity, were

subsequently equipped with

shearing/mixing sections [2].

One problem nevertheless

remained: very high pressures

at the end of the feed section

and, as a result, considerable

wear and tear on the screw and

barrel.

Fig. 1: Development of extruder screws in the USA and Europe

a = 3-zone comp ression screwb = Uniroyal screwc = Maillefer screwd = compression screw with

UCC(Maddock) mixere = compression screw with

pin mixerf = barrier screwg = barrier screw with UCC

(Maddock) m ixerh = 5-zone decompression screw

with shearing/mixing devicei = compression screw

with pin mixerk = no-compression screw with/

shearing/mixing devicel = barrier screw with

shearing/mixing device

m = high-output barrier screwwith shearing/mixing section


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3

The development of extruder

technology is basically reflected

in the development of the

screws . For thirty years,

development teams went their

different ways in the USA and

Europe until, at the beginning of

the nineties also due to

increasing globalization the

directions of development

began to converge again.

Combining the grooved bush

principle with barrier screws is

the logical step to optimize

extrusion technology [3].

Screw des igns and

se lec t ion c r i te r ia

As already mentioned, the

choice of a suitable extrusion

system (conventional or

grooved bush concept)

depends on the particular

application. After all, the

design of the screwdetermines the quantitative

and the qualitative properties

of the extrudate. In practice,

different screw lengths have

become established for

different applications. For

applications in extrusion blow

molding, for example,

relatively short extruders (L:D

= 20:25) are used, whereas

in other applications, such as

film and pipe extrusion,

extruders with longer screws

(L:D 30) are generally

employed. As a result, the

way the total screw length is

divided up into the "feed and

compression" and "melting

and homogenizing" sections

can vary considerably.

First of all, for a specific

application, a decision has to be

taken as to what proportion of

the total screw length should be

reserved for homogenizing the

plastificated melt. This question

can nowadays only be

answered on the basis of

experience or following an

appraisal of the demands made

on the melt quality. Even

specifying the necessary melt

quality can sometimes cause

problems. Complying with an

imprecisely defined melt quality

can necessitate not only

homogenizing elements on the

screw (dynamic mixing

sections), but also static mixing

elements.

The various constructions of

homogenizing elements will be

dealt with in more detail later.

While a wide variety of screw

concepts are still in use, current

developments are concentrating

very much on barrier screws.

For this reason, this report will

concentrate on such models

while taking a wider look at the

topic of single-screw extrusion.

Fig. 2 shows schematically the

basic concept of barrier screws

for different lengths of extruders,

with and without barrel venting.

The concept is the same for new

extruders as it is for the

retrofitting of existing machines.

The evaluation of a barrier

plastificating section is generallycarried out by looking at the

differences in the pitch and flight

depths and the design of the

feed section and outlet area of

the barrier flights. Both North

American and European barrier

Fig. 2 Basic concept of barrier screws


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screw developments have

moved in the direction of

designs which conform, to a

very large extent, to the princip le

of the Dray and Lawrence screw.

The characteristic features of

these screws are that, through

elevations in the respective

pitches of the main flight of the

screw and the barrier flight, a

sufficiently wide channel is

created in the solids channel

this encourages plastificating

and that, through a variable

adjustment of the flight depth

profiles, the melt temperature

curve can also be adjusted, with

the aim being to keep the melt

temperatures as low as

possible. Although barrier screw

designs still exist today with a

solids channel that is not sealed

off, the only way of ensuring

complete melting in the barrier

plastificating section is to use

solids channels with a 'dead-

end' groove (Fig. 3).

The front of the barrier flight at

the beginning of the barrier

plastificating section can be

designed with the melt channel

closed at the rear end or with an

open melt channel. In this case,

even if we assume that

unmelted material enters the

melt channel, complete

plastification is nevertheless

ensured by the end of the

barrier section because of the

long residence time in the melt

channel. A detailed description

of different barrier screw

concepts, including their

characteristic features, is given

in [4].

For extrusion applications in

which relatively high extrusion

temperatures are required (e.g.

paper coating), the screw

geometries must be modified by

making the flight depths in the

melt-filled sections smaller so

that, due to the higher

dissipation energy, the target

melt temperatures are reached.

This could possibly also be

done by adapting the feed

sections to reduce the specific

melt throughputs. Last but not

least, the shearing sections

used for such applications can -

and must - be dimensioned in

such a way that the necessary

temperature increase is

reached.

For other applications, for

example foam extrusion, exactly

the opposite course must be

taken to keep the melt

temperatures as low as possible

after injecting the b lowing agent.

Here, the best solution is to

regard the extrusion system as a

highly effective heat exchanger,

and to enhance its effectiveness

through reduced dissipation in

large-dimensioned screw

channels and through frequent

interface renewal by the screw

flights on the inner wall of the

barrel.

Fig. 3: Transition between feed section and barrier section on a doubleflighted/paired screw of 150 mm diameter


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We will now deal with the

influences of the raw material

and the extruder feed section

geometry, which determine the

conveying properties of an

extruder.

Universal screws / h igh-

ou tpu t sc rews

For the user, the ideal situation

would be to have a screw on

which as many different plastics

as possible could be processed

at high throughput speeds and

with good melt homogeneity.

Some of the most important

requirements are:

Processability of mixtures

with different sized and

different shaped granules

High plastificating

performance

Gentle but complete

plastification

Good melt homogeneity

Controlled melt temperatures

Minimal change in the

material through degradation

or crosslinking

High level of versatility: ability

to process a broad selection

of raw materials with a wide

range of throughput rates

Low performance-related

investment and operating

costs

In recent years, so-calledgrooved barrel extruders with

barrier screws have proved to

be the most suitable systems

among sing le-screw machines.

With many grooved barrel

extruders, the pressure build-up

at the end of the feed section is

too high, encouraging wear and

tear and impairing the stability of

the process. This can be

countered by enlarging the pitch

or making the screw channel

deeper, although this involves

the risk of plastification and

homogeneity problems.

A better solution than a screw

with a stepped pitch or channel

depth is a barrier screw. At the

beginning of the barrier

plastificating section, the

conveying flight changes to a

greater pitch; the beginning of

the barrier flight also has a

higher pitch. The depth of the

channels is adjusted to the

desired conveying and melting

characteristics. These two

measures result in a fairly

balanced, low pressure profile,

or even in a pressure build-up

towards the end of the screw

(Fig. 4).

When assessing the

"universality" of a screw, the

homogeneity of the melt plays a

dominant role. This is

particularly true when

processing mixtures of different

materials, and also with regrind

material, with color

masterbatches and with the so-

called 'direct extrusion' process

(combining compounding and

extrusion in one step, "in-line").

Barrier screws, too, must be

provided with elements for

homogenizing after the meltingsection. Depending on the

requirements of the raw

materials and the demands

made on the product, shearing

sections must be used for

dispersion (for example for color

pigments), and/or distributing

mixing elements must be

provided for axial and transverse

mixing.

0

50

100

150

200

250

300

350400

450

500

0 50 100 150 200 250

screw speed [rpm]

pressure[bar]

meltpressure in front of the screw tip

meltpressure after grooved feed bush

PE - HD Host alen GM 7746

Extruder 50 mm, 28:1 L/D

Fig. 4: Meltpressure in front of the screw tip and after grooved feed bush

(source: KKM)


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In practice, barrier screws with

neutral-pressure, (multiple-

)spiral shearing elements and

with faceted mixing sections

designed to give good flow

properties have proved

successful, also for direct

coloring with a color masterbatch

(Fig. 5).

With homogenizing elements of

this kind, the best way of

maintaining full control of the

general thermal conditions, and

thus of keeping the melt

temperature closely under

control, is to ensure that good

heat transfer by convection to

the temperature control system

of the extruder barrel is possible,

both in the area of the spiral

shearing elements and in the

area of the faceted mixing

sections, through constant

renewal of the surfaces and/or

interfaces between the moving

screw elements and the fixed

inner surface of the barrel. A

further influence can be exerted

on the temperature of the melt

by taking additional measures,

for example, by fitting a

temperature control system on

the inside of the screw (e.g. as a

closed cooling system),

As regards extruders for

universal applications - in other

words, extruders capable of

processing a very wide range of

raw materials, including regrind

(fluctuating proportions,

recycled material etc.) - a

decision has to be taken in each

individual case whether or not to

use the g rooved barrel extrusion

concept. The decision is not

always an easy one to make. If

excessively large variations in

the raw material properties

especially the bulk density, flow

properties and friction

coefficients are to be

expected, it is probable that

using the grooved bush concept

will lead to excessive

fluctuations in melt throughput

due to the fact that the output

characteristics are governed by

the solids conveying in the feed

section. In such cases a

smooth-bore extrusion system

can or must be used. This is

particularly true for processing

raw materials with a fairly high

shredded content and

consequently a low bulk density.

Another possibility is to pre-

compact the shredded material

so that grooved barrel machines

can function perfectly.

Fig. 5: Spiral shearing section and faceted mixing section after a barrier section

Homogen izing e lements Bar r ie r p las t i f i ca t ingsec t ion

Feed sect ion

Good d ispersive/distributive mixing effect

Heat transfer to the barrel

Low pressure loss

Effective separation of solidsfrom melt

High homogenizing effect

Good control of melttemperature

Clear pressure build-up

Melt throughput gearedto homogenizing capacity

Low pressure level

Low torque

Reduced wear and tear

Fig. 6: Barrier screw concept with homogenizing elements


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Fig. 6 summarizes the three

sections and shows the special

characteristics of a barrier screw

with homog enizing elements.

For universal application, use

can generally be made of

screws designed according to

the above concept in lengths of

between 20 and 30 x D. The

feed section consists either of a

shallow-flighted feed section

with subsequent decompression

(grooved bush concept) or a

constantly deep-flighted feed

section (smooth-bore extruder),

followed by the barrier

plastificating section and the

homogenizing sections.The best

solution is first to have

dispersively acting mixing

elements, and then distributively

acting elements. Fig. 7 shows

possible and commonly used

constructions.

In recent years, "static-dynamic"

mixer systems with spherical

indentations in a rotor and stator

have become quite popular.

They are marketed under such

names as CTM, TMR,

STAROMIX and 3-DD [5].

Although they have a good

mixing action, some of them

have an inadequate self-

cleaning system and others

have problems with wear and

tear. Apart from this, it is

essential that the melt is 100 %.

Opera t ion w i th bar r ie r

sc rews

With barrier screws, designed

according to the principle ofDray and Lawrence screws, the

solids channel has been made

wider. This provides a larger

contact surface area for the

material being melted so as to

introduce energy via the barrel

heating. This means that, with

barrier screws, the heating

process must be started

immediately after the material is

fed in. Either a constant

temperature program must be

set over the length of the barrel,

or the temperature must be set

so that it actually drops from the

feed section to the end of the

barrel.

The temperature at the end of

the barrel is the same as in a

conventional screw, in other

words it is geared to the melt

temperature. In the first heating

zone after the grooved bush, it is

perfectly in order to work at a

temperature which is about 20

30 C higher. In the lower to

medium speed range, the

temperature in the final barrel

section is set at the same level

as the melt temperature.

The temperature at the end of

the barrel is the same as in a

conventional screw, in other

words it is geared to the melt

temperature. In the first heating

zone after the grooved bush, it is

perfectly in order to work at a

temperature which is about 20

30 C higher. In the lower to

medium speed range, the

temperature in the final barrel

section is set at the same level

as the melt temperature.

With low-viscosity melts, or in

cases in which high melt

temperatures are required,

correspondingly higher settingsare recommended. It is also

advisable to keep a watch on

the relative periods in which the

heating and cooling units are

switched on (controller output

signals), so as to work in the

medium range of settings (20 ...

80 %). As a rule, this will mean

that the deviations between

target values and actual values

are sufficiently small to ensure

process stability.Fig. 7: Executions of shearing and mixing elements (source: KTP)


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In practice, it is not difficult to

establish the optimum

temperature settings.

Because of the special

characteristics of barrier screws,

it has often proved an

advantage to turn up the heating

output in the first and second

barrel sections, and to increase

the fan or cooling capacity in the

two sections at the end.

Prac t ica l exper ience

The broad range of application

of barrier screws for polyolefins

can be seen in Fig. 9. All these

materials were successfully

processed with the same 50mm/28 D screw with a twin-

spiral shearing section and a

faceted mixing section on a

grooved barrel extruder. Further

details are contained in one of

the later articles.

High-speed extrusion systems

are generally characterized by

the fact that the system is set upto suit a limited range of raw

materials, but so as to achieve

maximum melt throughputs in

the specified melt quality. For

this purpose, the combination of

a grooved barrel extruder and a

barrier screw with a

homogenizing section is

particularly recommended. With

larger screw diameters, a

double-flighted or twin-pair

screw system can be used to

improve the conveying

properties in the feed section

and to raise the melting

capacity.

One example at the upper end

of the speed scale involves

retrofitting an existing 150 mm

33 D extruder for working with

MDPE and LDPE for the

sheathing of steel pipes. The

objective in this case was quite

clearly to achieve maximum

possible melt output with high

product homogeneity (specified

in reference samples) and, at

the same time, to keep the melt

temperatures as low as

possible. In addition, the system

had to have outstanding self-

cleaning and material

changeover characteristics.

Fig. 8: Temperature program for b arrier screws

Fig. 9: Specific throughput vs. Screw speed (Quelle: KKM)


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Fig. 10 gives some examples of

results obtained with and

without a gear pump. Using this

concep t, the targets were readily

achieved, which meant that the

installed d rive power was almost

completely utilized for the given

range of raw materials. Further

increases in throughput are

conceivable over and above

these figures. However, this

would make it necessary to

adapt the drive unit by raising

the motor power and

proportionally increasing the

screw speed. It also becomes

increasingly important to take

special measures to prevent

excessive wear and tear

because of the greater influence

of the peripheral speeds of the

screw.

The production of fuel tanks is

an impressive example of the

direct recycling of production

scrap. A problem here is that,

with blow molding,

comparatively short extruders

are used.

Depending on the shape of the

tank and on other boundary

conditions, between 40 and 60

% flash occurs as scrap at the

production machine. This ismaterial which was cut off at the

top and bottom of the parison

and from the pinch-off edges of

the blow mould. This material is

directly ground and fed back

into the machine.

Fig. 10: Production data with extruder 150mm/33:1 for steel pipe coating


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Fig. 11 gives the key operating

data for a grooved barrel

extruder with a diameter of

150 mm/20 D equipped with a

barrier shearing/mixing section

screw for processing high-molecular weight HDPE grit

containing regrind material.

One notable application for

'specialty screws' are vented

extruders, which are used, for

example, in plants producing flat

film and sheets. Because such

machines are being increasingly

combined with melt pumps, thesecond stage of the screw only

needs to convey the melt

against the pump pre-pressure

(and possibly against the

resistance of melt filters), but

does not have to overcome the

resistance of the connection,

possibly a static mixing element,

and the die. Consequently,

much higher throughputs canbe achieved, with plastification

and homogenization being

carried out in the first stage of

the screw.

Fig. 12 shows the concept of a

vented screw with a barrier

plastificating section in stage 1,and a three-zone profile plus

faceted mixing section in stage

2. Such vented screws with

barrier plastification are being

successfully used for e.g.

polystyrene, polycarbonate and

PMMA.

Extruder 150 mm / 20 D

PE-HD Lupolen 4261A

with 50 % regrind

0

100

200

300

400

500

600

700

5 10 15 20 25 30 35 40 45

screw speed [r pm]

Throughput[kg/h]

190

195

200

205

210

215

Melttemperature[C]

[kg/h]

[C]

Fig. 11: Production data with extruder 150mm/20:1 for industrial blow moulding

Fig. 12: barrier screw configuration for vented extruder 90mm/30:1


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Extruder concepts for

d i f ferent p last ics / new

high-per formance

mate r ia l s

For most applications, grooved

barrel extruders with a heat

separation system between the

feed section and the subsequent

barrel have become established

in Europe. As a rule, the feed

bushes are axially grooved and

correspond to the construction

concept shown in Fig. 13.

A good thermal layout - in otherwords sufficient and uniform

heat dissipation in the area of

the grooved bush - is of

particular importance. For this

purpose, the cooling channels

and heat transfer resistances

must be optimized. In many

cases, temperature control units

or installations with bypass

control etc. are fitted to create

constant thermal conditions.

An optimized thermal layout is

also essential for the rest of the

barrel. Even though the target is

to generate as little excess heat

energy as possible via the

screw, it is not possible with

high-speed extrusion to

dispense with good cooling of

the barrel, at least not in some

sections. Special companies

offer heating/cooling

combinations for this purpose,

in which a great deal of heat can

be dissipated via aluminum or

copper elements. Some

machine manufacturers also

supply customized systems of

this kind. When new materials

come on to the market, the

question continually arises

about the most suitable extruder

and whether they can be

processed on existing

machines. This was the case

with LLDPE, and it is now

happening with the metallocene

polyolefins, for example

mLLDPE. The goal of the

chemical industry is where this

has not happened already to

make it possible to run mPE on

existing extruders by modifying

the molecular structure.

One aspect not being examined

here, but nevertheless of

considerable importance, is the

question of how the material

behaves in the dies and after

emerging from the die. With

blown film extrusion, for

example, this would concern the

melt elasticity and the bubble

stability.

D

N

DT

L

A

A

BH

H = 3 - 3,5mm

= 7 - 8L = 3 - 3,5 D

n = D(mm)/5D - D = ca.4mmB = 7 - 8mm

NT

Fig. 13 : Lay-out data for grooved feed sections


12/17

12

Basically, it can be said that

mPE can also be run on

extruders used for processing

LLDPE. This applies both to

grooved barrel machines andsmooth-bore extruders [6].

However, because of the

specific material properties,

there is a difference in the

throughput rates (Fig. 14) which,

in turn, leads to differences in

melt temperature (Fig. 15) and

outputs. On the other hand, this

phenomenon is not specific to

metallocene.

One of the most important

factors concerning the

conveying properties in the feed

section is evidently not the free

flowing characteristics or the low

degree of hardness of the

granules. In fact, the large

influence of lubricants as can

be seen in Fig. 16 indicatesthat friction on the surface of the

screw plays a major role,

something which has also been

encountered with "normal"

polyolefins (cf. Fig. 9). One way

of countering the poor

conveying characteristics of thesolid material is to add a

lubricant or material containing a

lubricant. Another possibility,

this time on the machine side, is

to reduce the coefficient of

friction by cooling the screw,

coating the surface etc.

Fig. 14: Specific throughput vs. Screw speed for grooved feed extruder 80 mm/30:1(source: Reifenhuser [6] )

Fig. 15: Melt temperature vs. Screw speed (source: KKM)


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For engineering plastics (i.e.

polyamides, polyesters,

polyurethanes or thermoplastic

elastomers), use is nowadays

predominantly made of smooth-

bore extruders. This is due not

only to "tradition", but also to the

predominantly low throughputs

involved. Plastics of this kind

can, however, also be

processed without problem on

grooved barrel extruders, as is

shown in [8]. For blow molding

with PA6, a grooved bush/screw

concept similar to the one used

for PE has given good results

[9].

Grooved feed sections are also

being increasingly used in

extruders with a barrel venting

system. The grooves with a

semi-circular, sickle, saw-tooth

or rectangular cross-section

are either cut in the one-part

barrel, or a normal grooved

bush concept is used. This

means that the construction of

vented extruders is currently in a

process of change, as was

already explained with the

barrier vented screws.

Direc t com pound ing in the

ex t ruder

This term is used to explain the

combination of compounding

and extruding in one step. The

process, which aims primarily to

cut down costs, is still in its

infancy, despite all the efforts

being made by machine

manufacturers and plasticsproducers. For processing, use

is made primarily of co-rotating

twin-screw extruders, which offer

far more possibilities in terms of

process technology. They can,

for example, be used for

incorporating fillers and

reinforcing agents, for blending

different polymers or for

simultaneously carrying out

reactions (reactive

treatment/extrusion).

There has, however, been no

lack of attempts to also use

single-screw extruders for

compounding or for the so-

called in-line extrusion. Systems

of this kind are repeatedly

shown and marketed. The

possibilities and limitations are

obvious.

One special kind of in-line

extrusion is the blow molding of

tubular film from mixed film

waste (DSD fraction). After a

temporary phase of euphoria,

normality has, however,

returned. Apart from technical

problems with individual

components of the plant and the

doubts about the product

quality, it has been found that

the cost structure is also

negative over the long term.

EXCEED in BlendsEffect of slip in the LDPE blend component on specific output

on grooved barrelextruders

2

2.2

2.4

2.6

2.8

3

3.2

3.4

80mm

24 L/D

grooved

75mm

25 L/D

grooved

80mm

30 L/D

grooved

Increase: 10 % 15 % 20 %

EXCEED

1MI /0.918D

EXCEED

1MI /0.918D

EXCEED

1MI /0.918D

+ 10 %

LDPE

+ 20 %

LDPE

+ 30 %

LDPE

with slip

Slip additivated LDPE as blend partner gives 10 - 20 % higher specificversus no slip in the LDPE

+ 5 %

LDPE + 5 %

LDPE

withslip

withslip

+ 5 %

LDPE

no slip

kg/rpm/h

Fig 16: Influence of slip agent on specific throughput with mPE (source: EXXON [7])


14/17

14

Despite all the progress being

made to adapt extruders and

extrusion lines to the

requirements of waste

processing, it must be said that

the production of extruded

quality products using recyclate

is limited. Not so much because

of the machine and processing

technologies, but more because

of the products and the

specified quality. For the time

being, in extrusion, the recycling

of production scrap will continue

to have priority over the

processing of post-consumer

recyclate.

Possib le appl ica t ions and

l im i ta t i ons

At this point, it should be said

that the possible range of

applications for grooved barrel

extruders with barrier screws is

almost "boundless". This is

shown elsewhere.

Retro f i t t ing to op t imize

the sys tem

Whenever funds for capital

expenditures become short, the

purchase of new machines

tends to be put back or

eliminated completely. In such

circumstances, optimizing the

existing system can be a help.

Where there is a need to modify

existing extrusion lines to cope

with a higher output and/orimproved melt quality, a modern

screw concept can be adapted

to the given circumstances.

When making a modification of

this kind, it first has to be borne

in mind that the machine in

question is of a given length

(e.g. frequently between 20 and

25 x D), which can not be

changed, and that it has an

existing drive unit. This

sometimes leads to restrictions

as far as the attainable specific

melt throughput is concerned,

due to bottlenecks with the

torque of the screw drive unit.

The torque results from the

installed motor power and the

installed gear reduction. In some

cases, the gear reduction can

be adapted so that a higher

drive torque is produced on the

screw shank. Since there is a

directly proportional relationship

between the specific output and

the screw drive torque, it is

possible in such cases to

achieve an increase in the

specific melt throughput equal

to the increase in torque.

Fig. 17 gives an example of a

successful retrofit.

Fig. 17: Production data with grooved feed extruder 60mm/24:1 after installing a barrier mixing screw(source: Kuhne)


15/17

15

Protec t ing the sc rew and

bar re l aga ins t wear and

tea r

One important aspect

concerned with protection

against wear and tear has been

discussed already, namely wear-

reducing screw geometries.

Quite astonishing results can be

obtained by harmonizing the

conveying characteristics and

the pressure build-up, and by

optimizing the melting process

from time to time in conjunction

with a multiple flight design.

With grooved barrel extruders,

for example, through optimized

harmonization of the system

(adapting the feed section

geometry to the downstream

sections and vice versa),

grooved bushes made of nitride

steel can be used instead of

hard metal or PM-HIP material

(see below), because of the

much lower pressures involved.

On the other hand, new plastics

- in some cases necessitating

higher processing temperatures

- fillers and reinforcing agents or

pigments, higher peripheral

velocities of the screw etc. etc.

are resulting in higher and

higher stresses. They can only

be countered by taking special

measures to increase the

protection against wear and

tear, as is state-of-the-art already

in injection molding [10].

In the USA, so-called bimetal

barrels have been used in

extruders for years and years. In

Europe, too, instead of the

conventional nitride steel

barrels, processors are making

increasing use of barrels that

have been given a centrifuged,

wear-resistant and, if needed,

corrosion-resistant armored

layer. Apart from the fact that

this approx. 1.5 2 mm thick

coat can be adapted to suit the

particular type of stress, it also

offers, with its consistent

properties, a certain "reserve" of

wear and tear, even if the

process engineering parameters

are not quite right.

For small and medium-sized

wear-protected screws (up to

approx. 50 mm), fully hardened

tool steel is used, particularly

cold work steel X 155 CrVmO

12.1 (DIN 1.2379). To achieve a

(limited) corrosion resistance,

frequent use is made of

rustproof, acid-resistant 17 %

chrome steel X 35 CrMo17 (DIN

1.4122). By ionitriding to

increase the surface hardness,

however, this material loses

some of its corrosion resistance.

For very high corrosion

protection, it is preferable to

choose special materials, e.g.

Inconel 625.

With larger screws, it is common

to armor-plate the screw flights,

which are particularly prone to

wear and tear. This involves

using the tungsten inert gas arc

welding or the plasma-powder

application (PPA) welding

method. The most popular

materials for this are nitride

steel, 30 CrMoV9 (DIN 1.8519)

and 14 CrMoV6.9 (DIN 1.7735),

or chrome steel X 35 CrMo17

(DIN 1.4122). Hard alloys such

as Stellite 12, Colmonoy 50,

Colmonoy 56, Colmonoy 83 etc.

are also used for armor-plating.

The screw root surface and

flanks can also be protected by

nitriding, by a hard chrome layer

or by armor-plating.

Hot isostatic pressed materials

(HIP) [10, 11] produced by

powder metallurgy are gaining

increasing importance. Both

"natural hard" and hardenable

alloys are used. The materials

can be produced either as

homogeneous systems or as

composite systems, in the latter

case, either in conjunction with

steel, e.g. as the core with an

external hard shell for screws or

screw elements, or as a

composite of the hard alloy

powder with inserted hard

substances.

The PM-HIP materials have the

advantage that a fine,

homogeneous and pore-free

structure is formed, which is

much preferable to the

conventionally produced

materials. The wear-inhibiting

hard phases (usually carbides)

are also distributed more finely

and evenly in the fine-grain

structure, which means that less

surface area is open to attack in

the matrix. The components can

be equipped specifically to cope

with the expected stresses.

Hardness values of up to 72

HRC can be attained. Fig. 18

shows the overall properties and

behavior of PM-HIP materials.


16/17

16

With materials examined on a

universal disk tribometer, it was

found that the relative wear

decreases significantly with an

increasing proportion of

vanadium carbide.

If we look at the market as a

whole, solutions in which PM-

HIP materials are used in single-

screw extruders are still the

exception rather than the rule.

The higher the demands made

on the plastics and their

additives, the more popular the

new systems will become, also

for these machines.

0

1

2

3

4

5

6

0 10 20 30 40

Vol.-% VC

Rel.Wear

Rel. Volumetric wear

Material Element [Gew.- %] VC [Vol.- %]C Cr V

X 220 CrVMoW 20 4 2,2 20 4,1 6,9

X 250 CrVMoW 22 6 2,5 21,6 6 10,3

X 260 CrVMo 26 4 2,6 26 4 6,2

X 270 CrVMoW 17 9 2,7 17 9 15,7

X 310 CrVMoW 15 10 3,1 15,2 10,3 18

X 340 VCrWMo 13 13 3,4 12,8 13,3 23,4

X 350 VCrMoW 13 9 3,5 8,5 13 22,8

X 380 VCrWMo 17 13 3,8 12,5 17 29,7

X 410 VCrWMo 17 14 4,1 14 17 29,5

X 450 VCrWMo 18 13 4,5 13 18 31,1

X 500 VCrWMo 20 13 5 13 19,5 33,4

Fig 18: Relative abrasive wear of PM-HIP materialsdependent from VC content (source: Reiloy)


17/17

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