2014 07 Kunststoffe Internationale FB Simulation Leichtbau

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INJECTION MOLDING
Lightweight Construction
[VEHICLE ENGINEERING] [MEDICAL TECHNOLOGY] [PACKAGING] [ELECTRICAL & ELECTRONICS] [CONSTRUCTION] [CONSUMER GOODS] [LEISURE & SPORTS] [OPTICS]
Faster to Series Production Through
Simulation
FRP Lightweight Engineering Based on Thermoset Matrix Systems Benefits from Integrated
Process Development
Aside from high part costs, investment and development costs often constitute an additional obstacle to the implementation of lightweight solutions in fiber-reinforced plastic composites. In particular, parts that are produced by reactive processing require an integrated approach to product and process development. Systematic
process simulation affords a way of considerably boosting efficiency during development.
Consistent lightweight construction: For the latch cover (small photo) of the X-Bow sports car, an
HP-RTM process was developed to near-series readiness (figures: KTM, Engel)
L
ightweight engineering with fiber-reinforced plastics (FRPs) imposes high demands on the development of new processes and parts: aside from the desired
weight savings relative to solutions in steel
or aluminum, it is important that cost-effi-
cient processes for mass production be
developed. However, this is difficult to
achieve because the parts can be complex. The very breadth of the range of materials involved complicates a purely empirical approach to process development.
Among the greatest challenges
faced today by such reactive processes
as high-pressure resin-transfer molding
(HP-RTM) are how to continuously lower
scrap rates and ensure consistently high
product quality. That is why, when the
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Kunststoffe international 7/2014
Lightweight Construction
materials and process parameters are
being selected, it is important that issues
such as downstream processing by
bonding or joining processes or surface
finishing be addressed. In addition, production-ready implementation of reactive processes is contingent on the availability of versatile yet customized automation.
An ever-increasing role is being
played by process simulation on the basis
of experimentally determined material
parameters. It can provide valuable contributions at nearly every stage of development that save time and costs and so
make the development process more
economical.
One example of this is the near-series
implementation of a high-pressure RTM
process for the manufacture of latch covers for the KTM X-Bow sports car (Title figure). The aim of this joint project by Engel
Austria GmbH, Schwertberg, and KTM
Technologies GmbH, Salzburg, both in
Austria, along with other companies, was
to implement the entire process workflow (Fig. 1), from choice of materials and
preliminary tests to mold design through
to parts manufacture in under six months.
The partner companies presented the results at the international plastics trade fair
K .
INJECTION MOLDING
Fig. 1. Even in the early development stages, simulation processes help to increase efficiency
(figure: KTM)
Fig. 2. A test platen
mold helps to experimentally validate
the filling simulation
(figure: KTM)
Material Selection:
Various Factors Are Decisive
The short duration of the project was
made possible by the use of simulation
methods at almost every stage of development. KTM Technologies performed
preliminary tests using a platen mold. Engel Austria and Hennecke GmbH, Sankt
Augustin, Germany, were responsible for
technical implementation of the process
and system engineering.
All projects commence by selecting
the materials with the specified properties of the end product in mind. In particular, mechanical parameters, raw material
prices, part weight and/or the desired
weight reduction relative to previous versions need to be considered. The choice
of manufacturing process also influences
the choice of material.
Mass production by means of HPRTM requires paying close attention to
the availability and processing of semi-finished textile goods. Here, the largest role
from the processing point of view is
played by characteristics such as ease of
draping and the permeability of the reinforcing materials, the viscosity, and the
tailored reaction profile of the reactive
matrix materials. When the materials are
being selected, it may well prove useful
to opt for a more expensive semi-finished
part if that will yield a stable manufacturing process with short cycle times. The
sometimes complex relationships between material properties and the costs
of the end parts will be discussed in detail
later.
Preliminary Trials:
Ensuring a Sound Data Basis
Preliminary tests on a trial mold are an essential part of process development. Experimentally determined process and
material parameters and variations thereof constitute a sound data basis for the inputs into the filling simulation, and afford
a way of realistically simulating processes
and validating the simulation methodology.
Standard practice here is to experiment on test platen molds. These are
used to evaluate various curing temperatures, process pressures and injection
speeds for different materials and to assess the impact on the resulting fiber-composite test specimen. In most
cases, it is already possible at this stage to
draw conclusions about the compatibility
of the binders used and the choice and
dosage of release agent.
The simulation method is then validated by conducting filling studies etc. to
compare simulated and measured pressure profiles with each other. Comparison
of the partial filling grade of a test platen
mold and the simulated flow front reveals
very good agreement (Fig. 2).
Darcy’s law states that the flow front
velocity ν is a function of the permea- »
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INJECTION MOLDING
Lightweight Construction
90
Practical Benefits
bar
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Pressure
Process simulation based on material parameters and preliminary experiments
contributed in this specific case to the development of an FRP-parts manufacturing
process using high-pressure RTM that was
both economical and was executed in the
comparatively short period of just six
months. In particular, employing the draping and filling simulation for the gate and
mold design in the early stages of production development yields a stable process
much faster, and avoids later modifications to the mold that would prove time
and cost intensive. Furthermore, the integration of the installation components
into a central control unit plays a key role
in achieving high process and quality assurance. This facilitates systematic control
and seamless documentation of the process parameters over the entire production process.
40
Pressure at gate, carbon
Pressure at outlet, carbon
Pressure at gate, glass
Pressure at outlet, glass
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10
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© Kunststoffe
Fig. 3. Pressure profile in the cavity for different reinforcement fabrics used to manufacture
test specimens: the lower permeability of non crimped carbon fiber fabric, compared with non
crimped glass fiber fabric, leads to a higher maximum pressure in the gate area (figure: KTM)
100
bar
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The Authors
70
Dr. Lorenz Reith is a development engineer in the Center for Lightweight Composite Technologies at Engel Austria
GmbH, St. Valentin, Austria;
[email protected]
Dipl.-Ing. Katharina Fischer works in
Research and Simulation at KTM Technologies in Salzburg, Austria;
Katharina.Fischer@KTM-technologies.
com
Dr. Michael Fischlschweiger is Head of
Development at the Center for Lightweight Composite Technologies at Engel;
[email protected]
Dipl.-Ing. (FH) Hans Lochner is Head of
Technology Development and Prototyping at KTM Technologies;
[email protected]
Dipl.-Ing. Peter Egger is Director of the
Center for Lightweight Composite Technologies at Engel; [email protected]
Service
Digital Version
B A PDF file of the article can be found at
www.kunststoffe-international.com/854654
German Version
B Read the German version of the
article in our magazine Kunststoffe or at
www.kunststoffe.de
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Pressure
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Pressure at gate, sheared
Pressure at outlet, sheared
Pressure at gate, unsheared
Pressure at outlet, unsheared
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© Kunststoffe
Fig. 4. Pressure profile in the cavity of sheared and unsheared textile reinforcing elements: at
any given geometry, shearing of the material leads to lower permeability and hence to higher
process pressures (figure: KTM)
bility K, the viscosity μ and the pressure
gradient ∇p. From
v=
[ K ] ∇p
µ
at a constant flow-front velocity, higher
viscosities and lower permeability
(less-permeable semi-finished fiber parts)
give rise to higher cavity pressures during
the injection stage.
The influence of different permeability values on the maximum pressure
during injection is clear: the lower permeability of the non crimped carbon fiber
fabric leads to a much higher maximum
pressure in the gate area than is the case
for the more permeable non crimped
glass fiber fabric (Fig. 3). But the experiments are not limited to studies of variations in reinforcing material. They can also
serve to estimate the influence of different lay-up sequences and/or fiber orientations.
It should also be noted that the cavity
pressure during the curing stage exerts a
decisive effect on the dimensional accuracy and/or the shrinkage compensation as
well as the surface finish. Generally, higher
pressures yield a better surface finish. The
process parameters determined from the
preliminary trials serve at a later stage as
the basis for commissioning the series-pro-
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Lightweight Construction
© Kunststoffe
Fig. 5. In addition to shearing of the material, draping trials serve to measure and visualize com-
pressions and flow-channel formation in the textiles
(figure: KTM)
INJECTION MOLDING
lation is to have high predictive quality.
For uniform progression by the flow front
and homogeneous part filling, there
must be no defects in the preforms. Such
defects in the final part can manifest
themselves on one hand as resin accumulations at radii and edges, and, on the
other, as localized areas of excessive fiber
content with insufficient infiltration.
The Institute of Textile Technology at
RWTH Aachen University, Germany, conducted draping trials using an optical
measurement system (manufacturer: Argus monitoring, Alsdorf, Germany). Transverse strain can serve to measure and visualize compression and flow-channel
formation in the textiles in addition to
material shear (Fig. 5).
Mold Concept: Avoiding the Need for
Downstream Optimization
Fig. 6. The manufacturing cell for producing the latch covers by the HP-RTM process was first
presented at K . The exhibit highlights the compact design of the v-duo machine
duction installation, and leads much more
quickly to a stable series process.
Preforming: Avoiding Defects
With regard to achieving fully automated
series production, not only the curing
time but more importantly the duration
and reproducibility of preform production are critical. A reproducible preform
process with stable fiber orientation and
constant permeability directly enhances
the reproducibility of the injection process and hence the properties of the
parts. Simply classifying the semi-finished
products on the basis of areal weight
would not be enough. Aside from the fiber volume content, the permeability of a
fabric depends on fiber shear.
(figure: Engel)
KTM Technologies studied the influence of fiber shear on the permeability
and the pressure profile by making sample panels from sheared and unsheared
semi-finished products. The pressure profiles that were recorded for the sheared
and unsheared semi-finished fiber parts
during the injection process clearly show
that shearing of material of given geometry lowers permeability and so increases
the process pressures needed for infiltration (Fig. 4).
As this shearing of the material is for
the most part also dependent on the
forming rate and has a direct impact on
local permeability, it is necessary to conduct a preliminary draping analysis and to
transfer the resulting fiber warpage and/
or the fiber orientation if the filling simu-
In the context of mold-concept evaluation, process simulation is an efficient way
to gain insights into the sometimes complex processes of infiltration of the
semi-finished part and of mold filling before the final design and manufacture of
production molds. KTM Technologies
uses PAM-RTM software program from
ESI Group, Paris, France, for this. The data
input required for the filling simulation
comes from process parameters such as
volumetric flow, and injection and curing
temperature in addition to the aforementioned material characteristics. The output values for this are obtained in the preliminary tests described earlier.
The surfaces of the part serve as the
geometric model, which is why the process simulation can yield information
about mold filling prior to mold design. In
particular, the dependence of part filling
upon various gate- or fiber-side sealing
strategies can be readily evaluated (Fig. 1).
From this, the optimal positioning of vent
units for avoiding air entrapment in the
finished part can be derived.
When combined with corresponding
preliminary experiments, simulation-assisted mold design, which already incorporates the draping and filling simulation,
proves to be particularly efficient. Costly
mold modifications are avoided. Validation of the models and parameters employed is provided by the technical implementation process. As a result, the
models used can be continuously developed and refined.
»
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39
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INJECTION MOLDING
Lightweight Construction
Fig. 7. All installation
parts are integrated
into the control unit
of the production
installation
(figure: Engel)
Series Implementation: Integration of
Control Unit Boosts Process Reliability
The near-series manufacturing process
for the X-Bow latch covers (Title figure) presented at K  used the polyurethane
system Elastolit (manufacturer: BASF Polyurethanes GmbH, Lemförde, Germany).
This material, which can be variously used
for high- and low-pressure RTM as well as
for wet pressing and spraying processes,
is characterized by its mechanical properties, which it imparts to the composite
material, e. g., high fatigue strength and
damage tolerance.
The mold was provided by Langer
GmbH & Co. KG, Illmensee, while the preforms came courtesy of Wethje GmbH
Kunststoff technik, Hengersberg, both in
Germany. The polyol and isocyanate
starting components were metered from
a Streamline high-pressure metering machine (Hennecke). This was equipped
with an MN-RTM mixing head which
has a cleaning piston with integrated displacement encoder for building up holding pressure.
The clamping unit (type: Engel v-duo
) was optimized for use with fiber
composites (Fig. 6). Its distinguishing features are a compact design and good accessibility. An Engel viper  linear robot
handled the preforms and removed and
stored the finished parts.
In the trade-fair exhibit, the metering unit was fully integrated in the new
CC control unit of the Engel machine. This ensures central processing
of all process parameters and generation of a consistent parts data record
over the entire process (Fig. 7). Integration of the control unit thus enhances
process reliability and contributes substantially to higher product quality. As
well as that, process optimization is
simplified, because the effects of
changes in process parameters can be
logged quickly and clearly. W
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