This document discusses the processing of two high-performance polyamides - a partly aromatic copolyamide (PA 6T/6I) and an amorphous homopolyamide (PA MACM 12) - using different screw designs in an injection molding machine. Standard three-zone screws and specialized barrier screws are evaluated. For the copolyamide, the barrier screw improves mechanical properties and does not damage glass fibers more than the standard screw. For the homopolyamide, the barrier screw provides better melt homogeneity. Overall, specialized barrier screws are recommended for processing demanding high-performance polyamides.

1. High-Performance Polyamides
Injection Moulding with Special Screws
Ralf Hala, Michael Kaisser, Domat/
Ems/Switzerland, and Nikolaus
Kudlik, NaÈfels/Switzerland
Highly exacting processing is required for novel high-performance poly-
amides for the production of injection-moulded parts for extreme applica-
tions. It is essential that the plasticizing unit prepares the material gently
and homogeneously and that all parameters affecting quality are observed.
The performance of polymer materials is
continually being extended and so too is
their range of applications. Novel polya-
mides which, on the basis of their prop-
erty profile, would rank above technical
thermoplastics and come within the
realms of high-performance polymers,
are increasingly superseding metals and,
as construction materials, opening up to-
tally new scope for innovative solutions.
However, the question repeatedly aris-
es as to how plastification in an injection
moulding machine must be designed and
arranged so as to yield moulded parts
whose quality is both high and consistent
with the property profile of the material.
Highly useful information has been re-
turned by extensive trials conducted
jointly by a raw materials producer and a
manufacturer of high-performance injec-
tion moulding machines on two materials
representative of this enhanced perform-
ance. The materials involved were a part-
ly aromatic copolyamide and an amor-
phous, aromatic, cycloaliphatic, transpar-
ent homopolyamide.
Partly Aromatic
with Demanding Properties
Copolyamide PA 6T/6I (grade: Grivory
HTV, manufacturer: Ems-Chemie, Domat/
Ems/Switzerland) is synthesised from
diamines and aromatic dicarboxylic
acids, a combination of terephthalic acid
(TPS) and isophthalic acid (IPS). Because
terephthalic acid is the predominant re-
agent, ISO 1874 refers to this copolya-
mide as PA 6T/6I. In the Anglo-Saxon
world, ASTM (American Society for Test-
ing and Materials) classifies such materi-
als as polyphthalamide (PPA) [1].
This partly aromatic copolyamide exhi-
bits unusually high performance proper-
ties: the glass transition temperature is
roughly 130°C and the melting point is
330°C. It has a high dimensional stability
and strength and is notable for its low
water absorption and good chemical re-
sistance. The outstanding feature that
sets it apart from most other polyamides
is the fact that its mechanical properties
undergo hardly any change when it ab-
sorbs water.
The primary application areas for this
high-tech construction material, which
can withstand short-term exposure to
300°C, are to be found in vehicle engine
compartments and in the electronics in-
dustry. PPA is virtually peerless in appli-
cations requiring dependable functional-
ity at high temperatures in hostile media.
One such example is the support element
from the brewing head of an espresso ma-
chine (Fig.1). Comparisons confirm that
PPA now rivals commercial die cast metal
alloys for strength [1].
Transparent, Highly Resistant
and Strong
The amorphous homopolyamide PA
MACM 12 is based on aliphatic and
cycloaliphatic monomers and is highly
transparent (grade: Grilamid TR 90;
manufacturer: EMS-Chemie, Domat/Ems/
Switzerland). Outstanding features are
its high ductility and flexural fatigue
strength. Its glass transition temperature
is 155°C.
This polyamide is notable generally for
its high chemical and weathering resist-
ance, high impact strength and stress-
cracking resistance. Even thick-section
parts are highly transparent. It is very
flame resistant and absorbs only minor
amounts of water in comparison to other
polyamides. Grilamid TR has so far main-
ly been used in the sanitary sector, for
personal hygiene products and for the
production of perfume containers ± any-
where that other transparent polymers
fail under a combination of high mechani-
cal stress and contact with chemicals.
The latest application is a turbo-washer
for milking machines (title picture).
On account of its very slight water ab-
sorption, PA MACM 12 has excellent di-
mensional stability and is destined for
the production of technically demanding
moulded parts.
Plasticizing Unit
Determines Quality
The plasticizing unit of an injection
moulding machine critically determines
the quality of the parts to be made. At the
same time, plastification should fulfil as
many as possible of the various demands
imposed by highly diverse materials dur-
ing processing. It must keep the melt
temperature within closely defined toler-
ances and require minimal heating
power. Furthermore, it should ensure
high throughput while maintaining good
melt homogeneity and applying low
shear. It should also be highly reproduci-
ble. Finally, wear must be slight.
Such diversity of tasks often pushes
the universality of standard screw per-
formance to the limit. In offering speci-
ality screws that are designed to process
certain types of materials or classes of
materials, machine manufacturers supply
solutions that will even yield good re-
sults with difficult materials and critical
moulded parts.
Because of the complex relationships
involved when plasticizing screws are
being developed, increasing use is being
made of simulation software. This makes
it possible to optimally adjust the geome-
try of the screw to the various intended
applications. Values predicted by simula-
tion software show good agreement with
trial and practice values.
The question will now be discussed as
to whether, in terms of mechanical char-
acteristics, homogeneity and economics,
a better alternative to the hitherto recom-
mended standard three-zone screw is
available for processing the high-perform-
ance polyamides Grivory HTV and Grila-
mid TR 90 presented above. The compari-
son is based on test specimens produced
on a high-performance injection mould-
ing machine (grade: SynErgy 800/230;
manufacturer: Netstal/Switzerland). The
Translated from Kunststoffe 90 (2000) 7,
pp. 82±85
Vol. 90 (2000) 7 33
HIGH PERFORMANCE PLASTICS
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2. screws used were a three-zone model, a
barrier screw (grade: HiMelt; Fig. 2) and
a low-compression screw (all models
made by Netstal/Switzerland).
In the case of Grivory, the object was
to study the strength and impact strength
as well as the influence of screw geome-
try on the degradation behaviour of the
material and on glassfibre damage. In the
case of Grilamid TR 90, the object was to
further enhance homogenisation and col-
our fastness.
Comparison of Screw Designs
A three-zone screw comprises a feed
zone, compression zone and a metering
or output zone. The ratio of screw volume
in the feed zone to the metering zone ad-
jacent to the compression zone is called
the compression ratio. The optimum com-
pression ratio differs from material to ma-
terial. The screw used here had a length
of 22.8 D and a diameter of 35 mm.
Low-compression screws are built on
the same principle as standard three-zone
screws (Fig. 3). The difference is that the
compression ratio is half as large.
Instead of having a compression zone,
the barrier screw (Fig. 4) has a barrier
land at the start of the barrier zone in the
melt channel. It divides the screw chan-
nel into a solid and a melt channel and
separates the melt from the solid. The
gap between the main land and the cylin-
der wall is dimensioned such that it can
only be passed by the melt and not by
pellet particles. The barrier land forms a
larger gap with the wall and has a much
higher slope than the main land. The van-
guard position of the barrier land serves
to enlarge the melt channel with increas-
ing screw length as progressive melting
occurs. At the same time, the width of
the solids channel decreases continuous-
ly. Ultimately, the barrier land meets the
main land and the compound bed be-
comes closed. Consequently, no unmelted
particles can enter the space in front of
the screw.
This geometry ensures that the pellet is
always compressed and pressed against
the cylinder wall, even when the size of
the solids bed decreases. The melt film
forming at the wall is continually scooped
over the adapted barrier land, and only a
thin layer of melt remains at the cylinder
wall. Thus, on the one hand, heat transfer
from the cylinder to the solid is enhanced
and, on the other, higher shear forces are
acting on the solid, generating locally
more heat for dissipation.
This effective incorporation of heat al-
lows the barrier screw to be so designed
as to considerably increase throughput
(Fig. 5). Nevertheless, the melt is exposed
to a lower shear stress because the melt
channel is cut more deeply than in a
three-zone screw. The fact that the melt is
treated gently permits low processing
temperatures, even in the case of high-
viscosity materials that would otherwise
lead to undesirable frictional heating and
overshooting of the set temperature in
conventional screw geometries.
Plasticizing Behaviour
of Various Screw Geometries
Material and thermal homogenisation of
the melt is crucial not only for transpar-
ent materials but also for technical mate-
rials. Judgements are difficult to make.
Generally, resort must be made to visual
assessment, a fact which means that pre-
cisely in the case of transparent materials
only a qualitative assessment can be
made. High plasticizing rates reveal the
limits of the three-zone screw in the form
of unmelted particles. Homogenisation
cannot be guaranteed, particularly in the
case of low-compression screws.
It is much easier to assess transparent
materials that have been slightly col-
oured. The colour distribution can be very
readily assessed visually and, in some
cases, may be measured mechanically.
Evaluation and Properties
of the Samples
The various screws were studied for their
effect on impact strength, tear strength
and elastic modulus. Also studied was
the extent of damage, if any, to glassfi-
bres. Another question of major interest
was whether the various screws degraded
the polymers.
The results of the extensive studies
confirm unambiguously that the standard
three-zone screw may be recommended
without reservation for the processing of
Grivory HTV.
Use of the barrier screw (grade: Hi-
Melt) improves the results of the mechan-
ical strength and toughness tests even
further. Thus the special screw consti-
tutes a potential quality factor for critical
moulded parts. The assertion that the bar-
rier screw causes up to 15% more dam-
age to the glassfibres than the three-zone
screw was disproved. The additional
screw flight and the degressive tempera-
ture profile do not appear to impair the
glass fibres.
Furthermore, it was found that neither
a high dynamic pressure nor a high
screw speed leads to chain degradation
within the polymer. Therefore, chain de-
gradation is dependent solely on the tem-
perature and time. As a process-related
increase in temperature is compensated
Metering zones Compression zone Feed zone
Temperature
Transport direction
Fig. 3. Construction and
function of a standard
three-zone screw
Melt Barrier land Granulate
Temperature
Main land
Transport direction
Fig. 4. Construction and
function of a barrier
screw
Vol. 90 (2000) 734
HIGH PERFORMANCE PLASTICS
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3. by cylinder heating, no problems can
arise from this angle either.
The standard three-zone screw also
yielded good results for the processing of
Grilamid TR 90. The quality of the mould-
ed parts can be influenced further dur-
ing the metering process via the dynamic
pressure dependency. Thus, the standard
screw constitutes a flexible solution with
which high-quality parts may be made re-
producibly.
The barrier screw offers slight advan-
tages in terms of homogeneity in this ap-
plication. This screw may be recommend-
ed for critical parts from the point of
view of colour distribution and proces-
sing window. Comprehensive trials on
other transparent materials confirm that
the barrier screw offers superior homoge-
nisation; this allows a reduction in the
use of masterbatches and hence helps to
lower costs.
Fig.1. Support element from the brewing head of an
espresso machine made from Grivory HTV
Fig. 2. Three-zone screw, built as a barrier screw
(grade: HiMelt; manufacturer: Netstal)
Fig. 5. Design of a barrier screw (D = 55; material:
PE-HD GF 4750)
Durchsatz = Throughput; Drehzahl = Speed; gemes-
sen = measured; gerechnet = calculated
Vol. 90 (2000) 7 35
COMPOSITES
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