Injection Moulding of Long-glassfibre-reinforced PP - Krauss Maffei

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I n
Injection Moulding of
ERWIN BÜRKLE
MATTHIAS SIEVERDING
JOCHEN MITZLER
automotive engineering, instrument
panels, front-ends and underbody
elements are increasingly being produced
from glassfibre-reinforced polypropylene.
PP is replacing engineering
plastics and metals from these applications
because of the lower density, cheaper
materials and recycling benefits. However,
PP can only meet mechanical specifications
if the reinforcement with long glassfibres
increases its elastic modulus and impact
strength.
The parts are made by either injection
moulding or compression moulding glassfibre-reinforced
PP. In compression
moulding, the starting material is usually
semi-finished panel goods made from PP
reinforced with glass mat thermoplastic
(GMT). Classic compression moulding of
GMT yields parts with excellent mechanical
properties because of the length and
isotropy of the fibres. Production of the
GMT is very complicated, though. The semi-finished
goods are therefore relatively
expensive.
Thanks to recent developments, it is
now possible to perform inline compounding
of PP and glassfibres followed
by direct compression. For all the
advances made in the process technology,
however, compression has major drawbacks
in comparison with injection
moulding. In most cases, the parts have to
be remachined. As a rule, openings in
compression-moulded parts can only be
effected with downstream stamping. This
generates production scrap and so adds to
costs.
Kunststoffe plast europe 3/2003
Injection moulding of long-glassfibrereinforced
parts from PP usually entails
processing long glassfibre granules with
the aid of modified plasticating units. By
way of alternative to the processing of
granules, Krauss-Maffei has launched a
new injection moulding compounder
(IMC) system, which manufactures injection-moulded
parts in a one-step process
direct from the PP and glassfibre base materials.
The injection moulding compounder
consists of a twin-screw extruder
and an injection-moulding machine
[1]. The extruder melts the PP and mixes
it with the glassfibres. The melt passes
through a buffer region into what is called
a shot-pot injection unit. From there, it is
injected into the mould. Compounding
directly during injection-moulding (IMC)
eliminates the need for a semi-finished
goods stage.
In the following, we compare the injection-moulding
machine and the injection
moulding compounder on the basis of
technical and economic criteria as an aid
INJECTION MOULDING ■
Long-glassfibre-reinforced PP
Process Comparison. Long-glassfibre-reinforced polypropylene parts are usually
made by injection moulding long glassfibre granules. A new one-step process
makes it possible to compound PP and glassfibres together for direct manufacture
as injection-moulded parts. Which of the two methods is better suited to a partic-
ular production task depends on the vagaries of parts production.
Translated from Kunststoffe 3/2003, pp. 47–50
Long-glassfibre-reinforced thermoplastics
Fig. 1. Processing methods for long-glassfibre-reinforced thermoplastics
for plastics fabricators to decide which of
the two methods is better suited to their
production tasks.
Behaviour of Fibre-reinforced
Thermoplastics
Good fibre/matrix adhesion is crucial to a
part’s mechanical properties. GMTs yield
somewhat higher strengths and impact
strengths than direct-processed moulding
compounds or long-fibre granules (Fig. 1).
The needle-punched mat structure, by
virtue of the physical anchoring of the fibres
and fibre filaments and the very good
i
Manufacturer
Krauss-Maffei Kunststofftechnik GmbH
D-80997 Munich
Germany
Phone +49 (0) 89/88 99-0
Fax +49 (0) 89/88 99-3092
www.krauss-maffei.de
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INJECTION MOULDING
PE 102542
filament distribution, offers advantages
which, however, compared with moulding
compounds injected either direct or via
long fibre granules, are lost if the flow
paths in the compression process are long.
The fact that injection moulding is better
at introducing fibre orientation into the
part can partly offset the disadvantage of
the absence of needle punching if the
design is suitable for the stress incurred.
Damage done to the fibre structure in
composites can be used to draw conclusions
about the processing method involved.
It may take the form of fibre breakage,
debonding and fibre pull-out. For full
use to be made of a fibre’s strength, it must
be longer than the so-called critical fibre
length lC.Corresponding literature values
for lC range from 1.3 to 3.1 mm for a
fibre/matrix laminate of PP and glass.
Use ofspecial coupling (size) can lead to
values of up to 0.9 mm.
The ratio of the current fibre length to
the critical fibre length can be used to infer
the quality of the fibre matrix coupling.
If the current fibre length in the part is
above the critical range, i.e. above lC, the
fibres can be expected to break. If it is lower
than the critical value, fibre pull-out can
occur. By that is primarily meant failure at
the fibre/matrix interface, as can happen
in chopped fibre compounds, where the
usual fibre length is 0.2 to 0.6 mm.
Strictly speaking, the length of the reinforcement
fibre remaining in the fibre is
of no relevance to the design. Mechanical
characteristics, such as strength, rigidity
and impact strength are more important
for the design of a part. Although they are
a function of the fibre length, their relationship
is highly complex. Analysing the
fibre length alone, therefore, can only lead
so far, although it is a practical parameter
for obtaining trend information. Figure 2
is a normalised diagram of the change in
rigidity, strength and impact strength as a
function of fibre length.
Fibre Length in the Part
When long-glassfibre-reinforced PP is being
processed, it is important for the
longest-possible fibres to be incorporated
into the part because that produces the
best mechanical properties in the composite.
However, there is no way of preventing
the fibres from breaking due to the
mechanical application of stress, and thus
being shortened, during compounding
and injection moulding. The greatest damage
to the fibres occurs while the melt containing
the fibre is filling the mould
(Fig. 3). Judicious mould design, howev-
er, can reduce the extent to which the fibres
are shortened. The melting process
greatly affects fibre length as well. There
are major differences between injectionmoulding
machine and injection moulding
compounder in this respect.
With the injection moulding machine,
the initial fibre length is restricted by the
size of the granules (as a rule 10 to 25 mm).
Manufacturers of long glassfibre granules
offer sheathed and pultruded systems
(Fig. 4). In pultrusion, the fibres are wetted
with matrix material in a melt bath and
joined together into bundles. This has the
advantage of impregnating the individual
fibres very evenly with matrix material. In
the case of the sheathed granules, the fibres
and the matrix material are coextruded together.
The melting process in the injection-moulding
machine has to dissolve the
fibre clusters and then wet the individual
fibres with matrix material (Fig. 5).
The extent of the damage done to the
fibres during melting decreases with decrease
in flow resistance. Large cross-section
flow channels are kinder to the fibres.
Screw configuration and the non-return
valve should therefore be modified correspondingly
when long glassfibre granules
are processed.
Fibre damage
When granules are injection-moulded,
the fibres are subjected to the complete
melting process. Mechanical stress on the
fibres lasts a relatively long time. The start
of plastication applies relatively large
forces to the fibres since at that stage the
matrix material has not yet completely
melted. Some of the fibres are trapped and
exposed to high shear forces.
The size and the metering stroke of the
screw additionally influence the fibredamage
mechanisms. A comparison of
Figures 3 and 6 shows that the large screw
with D = 165 mm causes much less damage
to the fibres than the small one with
D=90 mm. Figure 6 also illustrates the
negative influence of a longer metering
stroke (s/D = 1.5 to 2.5) on the remaining
fibre length. The scatter ranges show the
influence of the long glassfibre granule
structure (pultruded and sheathed).
By contrast, the injection moulding
compounder melts the pure matrix material
without fibres. The fibres are added to
the melt later and are thus exposed to
correspondingly less mechanical stress
(Fig. 7). This method is kinder than melting
in the injection-moulding machine
and leads to a higher average fibre length.
The injection moulding compounder
(IMC) offers the option of incorporating
endless rovings directly into the melt instead
of chopped strands. Although the
rovings are broken into shorter pieces by
the rotation of the screws, the resultant
fibres are relatively long on average (see
Fig. 3).
Economic Aspects
Fig. 3. Fibre lengths
as a function of the
processing section in
injection moulding
The price of the starting material is important
in the production of fibre-reinforced
PP parts. Long glassfibre granules
for injection moulding may be cheaper
than GMT semi-finished goods. Howev-
18 © Carl Hanser Verlag, München Kunststoffe plast europe 3/2003

PE 102542
er, fabricators have to pay more for the
granules than if they were to buy the individual
components separately. One of
the principal benefits of the injection
moulding compounder to fabricators is
that the starting materials are more economical
than the long fibre granules and
the cost of the materials diminishes as a
proportion of the cost of producing the
part.
Processing glassfibre-reinforced PP as
granules on an injection-moulding machine
is less capital-intensive than on the
injection moulding compounder. There is
the possibility, though, of modifying or exchanging
the plasticating unit on existing
injection-moulding machines so that they
can process long glassfibre granules. Even
if retrofitting is not possible and new machines
have to be installed, injection
moulding requires less investment. The
use of the additional twin-screw extruder
needed for the injection moulding compounder
complicates the equipment.
Decisions, Decisions…
Apart from the above-mentioned advantages
concerning fibre-length distribution
in the part, the injection moulding compounder
offers potential savings on starting
materials, but this potential can only
be realised through additional investment.
Important criteria for choosing between
an injection-moulding machine and the
compounder are therefore the weight of
the manufactured parts and the production
volume. The injection moulding
compounder has the edge if throughput is
high because the savings made at the time
the starting material is bought soon exceed
the higher capital outlay needed to buy the
equipment and so the investment soon
amortises. Where the parts are small or
production volumes are low, processing
long glassfibre granules on an injectionmoulding
machine can be the better alternative
because less capital investment is
needed.
The injection moulding compounder
confers production flexibility on fabricators,
allowing them to tailor the materials
Kunststoffe plast europe 3/2003
to their needs. They can selectively modify
the matrix/fibre/size system such that,
for example, the fibre content in the part
exactly matches the respective technical
specifications. When granules are being
processed, this selective modification is
only possible under certain conditions because
the manufacturers only offer granules
with certain fibre contents. To change
the fibre content for classic injection
moulding, fabricators must blend the long
glassfibre granules with unreinforced PP
– a working step which makes additional
demands on the machine and the material
supply systems.
However, the degrees of freedom that
the injection moulding compounder confers
on fabricators in terms of material
composition increase product accountability
and thus the liability of fabricators.
Fabricators must now assume responsibility
for the quality assurance and the
guarantees hitherto borne by the granule
manufacturers. But therein lies a major opportunity
for the future. The IMC process
greatly increases the added value created
by fabricators.
Summary
Long-glassfibre-reinforced parts made
from PP can be produced with an injection
moulding compounder or with a
modified injection-moulding machine.
Pronouncements as to which process is
more economical can only be made after
the specific boundary conditions have
been considered. The advantages of the injection
moulding compounder – cost savings
in starting materials, gentler fibre
treatment and longer fibres – primarily
come to bear when in high-volume production
and when the parts have to meet
very high mechanical specifications. With
smaller parts and lower outputs, the more
favourable option may be to process longglassfibre
granules on conventional injection-moulding
machines. ■
THE AUTHORS
DR.-ING. ERWIN BÜRKLE, born in 1942, is head of
pre-development, new technologies and process
INJECTION MOULDING ■
technology in injection moulding at Krauss-Maffei
Kunststofftechnik GmbH, Munich.
DIPL.-ING. MATTHIAS SIEVERDING, born in 1970,
works in the Applied Technology/Development department
for IMC (injection moulding compounders)
at Krauss-Maffei.
DIPL.-ING. JOCHEN MITZLER, born in 1973, works
in applied technology/development of new processes
for injection moulding machines at Krauss-Maffei.
Fig. 2. Mechanical properties: Mechanical
properties as function of fibre length
E-Modul = Modulus of elasticity; Festigkeit =
Strength; Schlagzähigkeit = Impact strength
Faserlänge = Fibre length; Kurzfaser-Bereich =
Range of short fibres; Langfaser-Bereich =
Range of long fibres
Fig. 4. Long-glassfibre-reinforced granules:
Development from short fibre granules to long
fibre granules
Kurzfasergranulat = Short-fibre granules;
Faserlänge = Fibre length; Ummantelung =
Sheathing; pultrudiertes Langfaser-Granulat =
Pultruded long-fibre granules
Fig. 5. Melting process: Melting of long fibre
granules in an injection-moulding machine
Faserschonende Strömung = Flow that is kind to
fibres; Faser-Cluster-Auflösung = Dissolution of
fibre clusters; Einfüllen und Einziehen in den
Schneckenkanal = Filling and feeding into the
screw channel
Fig. 6. Fibre damage: Influence of the metering
path s on the fibre-length distribution after the
screw for long-fibre granules PP 30 GF (Lf =
10–12 mm) in an LF special screw where D =
90 mm
gewichtete Häufigkeit = Weighted frequency
Faserlänge = Fibre length; unter = under; über =
over; davon über 5 mm = Of which over 5 mm
Fig. 7. Mode of operation of the injection moulding
compounder: Direct processing of glassfibre
rovings on the injection moulding compounder
Glas = Glass; Plattformwaage = Platform balance
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