Fiber-Reinforced Thermoplastics as Sandwich

Transcription

INJECTION MOLDING
Fiber-Reinforced Thermoplastics
as Sandwich Construction
Lightweight Construction for Mass Production. Innovative production methods
permit load-oriented lightweight structures to be manufactured in sandwich construction. The potentials of this method are extended by efficient processes suited
for volume large series.
ALEXANDER ROCH
ANDREAS MENRATH
TIMO HUBER
andwich construction combines
lightweight material construction
with lightweight design, thereby
merging high stiffness and strength – in
relation to weight – with additional functions such as thermal and sound insulation. The face sheets of the composite material absorb bending moments in the
form of tensile stress and pressure. Hereby, the core has the task of bonding the
two face sheets firmly and with high shear
resistance. The resulting variability can
cover a wide spectrum of mechanical,
thermal, and acoustic properties. Such a
S
Translated from Kunststoffe 10/2013, pp. 183–189
Article as PDF-File at www.kunststoffeinternational.com; Document Number: PE111514
Kunststoffe international 10/2013
compound exhibits specific properties
that cannot be obtained with a monolithic approach. Because of the high manufacturing costs, structure-optimized
sandwich components were initially used
only in the aerospace industries. New,
more cost effective manufacturing procedures have now promoted the industrial
proliferation of sandwich constructions.
With foam injection molding (FIM), a
propellant is worked into the thermoplastic melt, distributed as homogeneously as
possible, and dissolved. The aim is always
to create a single-phase gas/polymer solvent in the metered volume in front of the
screw tip, i.e. to completely dissolve the
propellant in the melt. Hereby, the solubility depends greatly on system pressure
– the higher the back pressure, the more
propellant fluid can be dissolved theoretically in the melt. During injection into
the cavity, a rapid drop in pressure occurs,
which causes the molding material to
foam. This in turn results in the formation of an unfoamed, sealed outer layer
on the molded part, and a foamed core.
In simple terms, the mechanical properties of such a structure can be compared
with those of a sandwich or a T beam
(Fig. 1).
>
Fig. 1. The working principles of polypropylene
integral foam can be compared with a sandwich structure or a T beam (figures: Fraunhofer ICT)
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119
INJECTION MOLDING
Advantages of
Foam Injection Molding
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
Fig. 2. All LFT-D
foamed test samples
have the same
weight as the compact reference components (left). Wall
thickness and density
increase from left to
right
While the maximum possible density
reduction with partial filling is very limited (usually about 10 %), two other approaches for saving material and weight
are highly promising:
Thin-wall injection molding: Due to
the lower viscosity and the possibility
of injecting “from thin to thick”, many
components can be built thinner and
thereby lighter.
Negative compression: After injection
with a narrow mold gap, the cavity volume is enlarged, which greatly increases the maximum possible density reduction.
This article will deal with the enormous
potential of the latter process (also known
as breathing mold or precision opening)
in combination with long fiber and continuous-fiber reinforcement.
Three Versions for LFT Foams
In extensive investigations, researchers at
the Fraunhofer Institute for Chemical Tech-
nology (ICT) in Pfinztal, Germany, have
produced and characterized long-fiber-reinforced thermoplastic foams (LFT foams).
Using negative compression, and by varying wall thickness, test samples were produced with PP-LGF30 that have the same
weight,but exhibit different densities (Fig. 2).
In other words, the same amount of longfiber-reinforced melt was injected into the
shear edge mold,and the cavity subsequently enlarged.Simultaneously with the mold's
expansion stroke, the foam pressure enabled the melt to expand to the desired final dimensions. Hereby, three different
foaming processes were applied:
Long-fiber pellets (LFT-G) with a
chemical propellant (CBA) and LGF
screw (105 mm);
Long-fiber pellets (LFT-G) using the
MuCell process with LGF screw
(80 mm) and nitrogen (N2) as propellant;
LFT-D foaming process with an injection molding compounder (IMC) and
nitrogen (N2) as propellant.
6.5
H = 5.6
LFT-G CBA
LFT-G MuCell
LFT-D foam
H = 5.7
kg/m2
H = 5.1
H = 5.2
Surface weight
Relative density-specific
bending stiffness
With the FIM process, a basic distinction
is made between chemical and physical
propellants (info box). The propellant significantly lowers the melt's viscosity, so
that lower injection pressures are required, and longer flow paths are possible. As a rule, in order to achieve a weight
reduction compared with compact injection molding, the cavity is only partially
filled – the remaining volume is filled by
the generated foam pressure. In this way,
the internal mold pressure is also reduced,
so that lower clamping pressures are possible. The foam pressure acts homogeneously and in all areas of the cavity, so
that there is no need for a holding pressure phase. This leads to lower internal
stress and less distortion, which clearly
improves the dimensional accuracy of the
molded part.
What is more, without a holding pressure phase, and due to the homogeneous
foam pressure in the cavity, it is possible
to inject “from thin to thick”. A circumstance that can offer enormous advantages for a “foam-compatible” mold design. With thin-walled components, the
overall cycle time is frequently reduced,
in particular because the foam provides a
very good contact to the cavity's cooling
wall.
A further advantage is the great freedom of design. With foam injected components, the limits of “plastic-compatible” designing are partially eliminated.
For example, components with very thick
walls (up to several cm) or with large wall
thickness variations (e.g. 1:10) can be
produced without sink marks.
H = 4.8
H = 4.3
H = 4.1
H = 3.7
5.5
H = 4.6
5.0
4.5
H = 4.2
H = 4.7
Δm ≈ 35 %
Δm ≈ 27 %
Δm ≈ 21%
4.0
H = 3.6
LFT-D compact
LFT-D foam
3.5
0
5
10
15
20
25
Density reduction Δρ
30
35 % 40
3.0
1.0
1.4
1.8
2.2
2.6
3.0
3.4
3.8
Relative density-specific bending stiffness
4.2
© Kunststoffe
Fig. 3. With all the described foaming processes, bending stiffness increases significantly with an increasing reduction of density Δρ or final wall thickness H (left)*. In order to obtain the same respective bending stiffness, more surface weight must be used for the compact method than with LFT-D foam
(right)
* Important: The diagram at left does not permit a comparison of the processes. Only the characteristic values of the same process can be compared
120
© Carl Hanser Verlag, Munich
Relative density-specific perforation energy
INJECTION MOLDING
!
1.2
1.0
0.8
0.6
0.4
LFT-G CBA
LFT-G MuCell
LFT-D foam
0.2
0
0
5
10
15
20
25
Density reduction Δρ
30
35
%
40
© Kunststoffe
Fig. 4. Also with high density reductions, the penetration energy with LFT foams is maintained
With all three processes, density could be
reduced by up to 37 % without problems.
Compact, unfoamed test samples served
as references. Particularly with large-surface components, negative compression
offers advantages: All investigated foaming processes exhibited significant gains
in bending stiffness with increasing density reduction Δρ and final wall thickness
H (Fig. 3, left). With a final wall thickness of
e.g. H = 5.6 mm, the bending stiffness of
the sample foamed with chemical propellant was about 4.3 times higher than that
of the compact reference component.
This increase is due to the geometrical dependency of the area moment of inertia,
which increases with the cube of wall
thickness.
A comparison with LFT-D compact
injection molding illustrates the weight-
saving potential of the LFT-D foam
(Fig. 3, right): To achieve the same bending
stiffness, a higher surface weight is required for the compact version than with
foaming. In order to further increase this
lightweight construction effect, injection
can be done with an even smaller initial
cavity volume, and/or the cavity volume
can be enlarged even more until the
maximum possible final wall thickness
is obtained (material and process dependent).
With negative compression of unreinforced polymers, a significant embrittlement occurs. However, such a behavior was not observed for the investigated LFT foams: With every process, and
for every density reduction, the penetration energy remained at a comparable
level (Fig. 4).
Process Technology
The FIM process distinguishes two versions. Chemical propellants are admixed to the base polymer in the form of
powder or as pellets during processing.
Above certain process temperatures, the
propellant decomposes and splits off gases that are dissolved in the polymer melt.
Apart from these gaseous propellant fluids, solid decomposition residues are created during reaction, whose compatibility
(e.g. color, corrosion, smell etc.) must also
be taken into account when selecting the
propellant. In principle, the use of chemical propellants is possible on standard
injection molding machines that are
equipped with a screw position controller
as well as a sealing nozzle for the barrel
and/or the mold.
Physical propellants are gases, mostly nitrogen (N2) or carbon dioxide (CO2),
which are introduced directly into the
plastic melt, i. e. without the “detour” of a
chemical decomposition reaction. For this
purpose, a gas metering device is required, which admittedly makes plant
technology more complex, but is able to
dissolve large amounts of gas in the polymer, thereby “boosting” the numerous advantages during foaming.
Sandwich Structures with
Continuous-fiber-reinforced
Face Sheets
For a further increase in performance, the
combination of continuous-fiber com- >
INJECTION MOLDING
E-modulus
Unidirectional
fibers (UD)
Fig. 5. Qualitative
representation of the
elastic modulus with
different types of
reinforcement, as a
function of load
angle ␤
Mat
β = 45 °
Pure
matrix
Fabric
E-modulus
© Kunststoffe
posite semi-finished products and FIM is
conceivable, e.g. in a sandwich structure
with continuous-fiber-reinforced face
sheets and injection-moldable core layer.
The injection-moldable area is foamed
with a propellant, and can then serve e.g.
for functional integration (inserts, snap
fittings) and for implementing complex
geometries (webs etc.). The continuousfiber-reinforced areas serve for force application or transmission, and provide the
necessary component stiffness.
Combination of continuous-fiber
composite semi-finished products and
the FIM process allows cost-effective, automatable manufacture of sandwich
structures consisting of pre-consolidated
and fiber-reinforced face sheets with a
foamed core. Bonding of the face sheets
is done in a single processing step by introducing the core material with subsequent foaming.
The mechanical properties of the face
sheets are determined by the properties
of the fibers and the matrix material, the
volume percentage of the fibers, and fiber
orientation. The range of continuousfiber composite semi-finished products
extends from chopped strand mats
through fabrics up to laminates. These
laminar semi-finished products are distinguished mainly by their fiber orienta-
122
tion and fixing, and cover the entire bandwidth from isotropic to anisotropic material behavior (Fig. 5). For maximum utilization of the fiber properties, the load
angle relative to fiber orientation is decisive. Deviations of just a few degrees result in a large decrease of the mechanical
properties.
Continuous-fiber-reinforced thermoplastics exhibit a lower tendency to creep
and a higher heat distortion temperature.
Apart from technical considerations, sustainability aspects should also be mentioned in the interests of a holistic view.
Fiber-reinforced thermoplastic material
systems are featured by a closed material
cycle.
A polypropylene (PP) based material
system was selected to manufacture the
sandwich moldings. For reinforcerment,
unidirectional (UD) oriented PP/GF tapes
were applied in an automated process, and
then consolidated completely. This procedure is suitable for manufacturing face
sheets with different fiber orientations and
any number of layers as well as hybrid
laminates. In this way, the reinforcing
structure can be perfectly matched to the
respective loading situation, and thereby
used efficiently. As core material, a PP
compound with optimized processing
and foaming properties was used.
INJECTION MOLDING
Fig. 6. Schematic diagram of the processing steps for manufacturing thermoplastic sandwich structures with continuous-fiber-reinforced face sheets,
starting with fixing the fiber-reinforced face sheets in the cavity (A), and ending with possible trimming of the component (F)
The production of foamed sandwich
sheets with continuous-fiber-reinforced
face sheets using the FIM process is divided into several basic processing steps
(Fig. 6). The consolidated face sheets are
fixed on the opposite sides of the mold
cavity (A). Depending on the material
and thickness of the face sheets, the bonding surface is heated indirectly (B). Subsequently, the mold is closed as quickly as
possible in order to minimize the temperature loss. Injection of the gas-charged
melt is superimposed with a compression
stroke of the shear edge mold. This en-
sures a uniform pressure distribution in
the cavity, and improves bonding as well
as the tendency for distortion (C). After
volumetric cavity filling has been concluded, an expansion stroke (negative
compression) creates a pressure drop. The
sandwich core is foamed until the required component dimensions have been
obtained (D). When the cooling time has
elapsed, the component is removed (E).
Depending on mold geometry, subsequent trimming might be necessary (F).
Implementation of this process is conceivable for all laminar semi-finished
fiber products. The potentials become apparent when comparing the specific component stiffnesses of UD-reinforced sandwich components with constant final wall
thickness and two face sheet thicknesses
with those of compact, foamed and unreinforced reference components (Fig. 7).
Component stiffness is increased clearly by the face sheets. With a final wall
thickness of H = 6.4 mm, sandwich
structures with face sheets reinforced with
0.26 mm diameter glass fibers exhibit a
12.7 times higher specific component
stiffness than the compact references. >
www.kunststoffe-international.com
123
INJECTION MOLDING
Specific component stiffness
[N/mm×cm3/g]
1,200
Description
1,000
Face sheet
material
Raw density
reduction Δρ
[%]
Percentage of
face sheet
volume k
[]
0
0
–31.1
0
–22.1
0.08
2.6
0.31
800
Compact reference
(CR)
600
400
Foamed reference
(FR)
200
Sandwich component
(UD-GFV 0.26 mm)
0
Compact
reference
Foamed
reference
0.26 mm
1.00 mm
Unidirectional glass-fiber
reinforced face sheets
–
UD-oriented PP/
GF laminates
Sandwich component
(UD-GFV 1.00 mm)
© Kunststoffe
Fig. 7. Specific stiffness of compact, foamed and fiber-reinforced components. All test samples have the same final wall thickness
With a face sheet thickness to 1 mm, the
specific component stiffness increases to
a value 20 times higher than that of the
compact reference.
For economic use, particularly in
structural components subjected to
bending stress, use of the continuousfiber-reinforced material in the areas of
maximum stress is a prerequisite. By varying the face sheet thickness as well as the
volume ratio between face sheet and core
layer, it is possible to adjust the mechanical properties in an area between the
characteristics of the foamed core material and those of the homogeneous laminate.
Conclusion
Fiber-reinforced sandwich structures
produced with foam injection molding
exhibit high lightweight construction potential for future series-produced appli-
124
cations. The molding techniques of negative compressions and back injection are
particularly suited for laminar items such
as under-floor structures, door modules,
pallets, containers etc. High values for
bending stiffness and good impact
strength are demanded for these products. The described material and process
combinations meet these requirements
completely. REFERENCES
1 Altstädt, V.; Mantey, A.: Thermoplast-Schaumspritzgießen. Carl Hanser Verlag, München 2011
2 Spörrer, A.: Leichte Integralschäume durch
Schaumspritzgießen mit optimierten Werkstoffen
und variothermen Werkzeugen. Dissertation, Universität Bayreuth 2010
3 Müller, N.: Spritzgegossene Integralschaumstrukturen mit ausgeprägter Dichtereduktion. Dissertation, Universität Erlangen-Nürnberg 2006
4 Roch, A.; Huber, T.; Henning, F.; Elsner, P. : LFTFoams – Lightweight Potential for Structural Com-
ponents Through the Use of Long-Glass-Fiber-Reinforced Thermoplastic Foams. 29th PPS Conference, 15.-19. Juli 2013, Nürnberg
5 Henning, F.; Moeller, E.: Handbuch Leichtbau. Carl
Hanser Verlag, München 2011
6 Schürmann, H.: Konstruieren mit Faser-KunststoffVerbunden, Springer Verlag, Heidelberg 2005
7 Ehrenstein, G. W.: Faserverbund-Kunststoffe. Carl
Hanser Verlag, München 2006
THE AUTHORS
DIPL.-ING. ALEXANDER ROCH, born 1983, is scientific assistant at the Fraunhofer Institute for Chemical Technology (ICT) in Pfinztal;
[email protected]
DIPL.-ING. ANDREAS MENRATH, born 1985, is
scientific assistant at the Fraunhofer ICT;
[email protected]
DIPL.-ING. TIMO HUBER, born 1981, is group
leader for thermoplastic processing at the Fraunhofer
ICT; [email protected]

Fiber-Reinforced Thermoplastics as Sandwich

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